Synchronous remote replication of snapshots

ABSTRACT

Snapshots from a first LSU (R 1 ) on a first storage system (A 1 ) may be replicated to a second replica LSU (R 2 ) on a second storage system (A 2 ), for example, concurrently to remotely replicating (e.g., synchronously) write operations for R 1  to R 2.  A process, P, on A 1  executing the replication of the snapshots from R 1  to R 2  may be a separate process than the one or more processes on A 1  executing remote replication of write operations for R 1  to R 2.  During a consistency window on A 1,  outstanding write operations for R 1  at the time the consistency window opened may be logged, and a pair of snapshots, SS 1   1  and SS 1   2  may be activated on R 1  and R 2,  respectively. After the consistency window has closed, the SS 1   2  snapshot metadata and snapshot data may be updated based on the outstanding write operations.

BACKGROUND Technical Field

This application generally relates to data storage networks, and moreparticularly to remotely replicating snapshots from one storage systemto another storage system on a storage network.

Description of Related Art

Data storage systems (often referred to herein simply as “storagesystems”) may include storage resources used by one or more host systems(sometimes referred to herein as “hosts”), i.e., servers, to store data.One or more storage systems and one or more host systems may beinterconnected by one or more network components, for example, as partof a switching fabric, to form a data storage network (often referred toherein simply as “storage network”). Storage systems may provide avariety of data services to host systems of the storage network.

A host system may have host applications that utilize the data servicesprovided by one or more storage systems of the storage network to storedata on the physical storage devices (e.g., tape, disks or solid statedevices) thereof. For a given application, to perform input/output (I/O)operations utilizing a physical storage device of the storage system,one or more components of the host system, storage system and networkcomponents therebetween may be used. The one or more combinations ofcomponents of the host, switching fabric and storage system over whichI/O operations between an application and the storage system may becommunicated may be considered an I/O path between the application andthe storage system. It should be appreciated that other combinations ofcomponents of a storage network, for example, two or more storagesystems, also may be coupled together by one or more switches of aswitching fabric. Thus, more generically, the one or more combinationsof components of a first network component, switching fabric and secondnetwork component over which I/O communications may be communicated maybe considered an I/O path between the two network components. Thecollective I/O paths between components of a storage network may beconsidered to define a connectivity of the storage network.

Host systems may not address the physical storage devices of a storagesystems directly, but rather access to data may be provided to one ormore host systems from what the host system(s) view as a plurality oflogical storage units (LSUs) including, for example, logical blocks,logical devices (also referred to as logical volumes, LUNs and logicaldisks), thin devices, groups of logical devices (e.g., storage groups),NVMe namespaces, and other types of LSUs. LSUs are described in moredetail elsewhere herein.

SUMMARY OF THE INVENTION

In an embodiment of the invention, a method may be performed for asystem including a host system, a first storage system, a second storagesystem, a first logical storage unit for which data is replicated fromthe first storage system to a second logical storage unit of the secondstorage system, a method of remotely replicating a first snapshot of thefirst logical storage unit from the first storage system to the secondstorage system. The method includes: at a first point in time,suspending initiating of processing on the first storage system of newwrite requests for the first logical storage unit received from the hostsystem after the first point in time; recording at least a firstoutstanding write request for the first logical storage unit on thefirst storage system that is outstanding at the first point in time;activating the first snapshot on the first storage system; initiatingactivation on the second storage system of a second snapshot of thesecond logical storage unit as a remote replica of the first snapshot;after the recording, the activating and the initiating: resuming theinitiating of processing of new write requests for the first logicalstorage unit on the first storage system, and applying the at leastfirst outstanding write request to the second snapshot. The applying maybe performed as part of a process executing independently of, andconcurrently with, one or more processes that are remotely replicatingwrite operations of the first logical storage unit to the second logicalstorage unit. Applying the at least first outstanding write request mayinclude, for each outstanding write request of the at least firstoutstanding write request: determining whether the outstanding writerequest requires modification of snapshot data of the second snapshot;and, if it is determined that the outstanding write request requiresmodification of the snapshot, modifying the snapshot data and snapshotmetadata of the second snapshot on the second storage system based onthe outstanding write request. The first outstanding write request mayspecify a write operation for a first logical storage elementcorresponding to a second logical storage element of the second logicalstorage unit, and determining whether the first outstanding writerequest requires modification of snapshot data of the second snapshotmay include: determining whether first snapshot metadata of the firstsnapshot references a first current value of the first logical storageelement on the first storage system; determining whether second snapshotmetadata of the second snapshot references a second current value of thesecond logical storage element on the second storage system; anddetermining whether to modify the snapshot data and snapshot metadata ofthe second snapshot based at least in part on whether the first snapshotmetadata references the first current value and whether the secondsnapshot metadata references the second current value. Determiningwhether the first outstanding write request requires modification ofsnapshot data of the second snapshot may include: determining that thefirst snapshot metadata references the first current value of the firstlogical storage element; determining that the second snapshot metadatadoes not reference the second current value of the second logicalstorage element; and modifying the snapshot metadata of the secondsnapshot to reference the second current value of the second logicalstorage element. Determining whether the first outstanding write requestrequires modification of snapshot data of the second snapshot mayinclude: determining whether a first value of the second logical storageelement specified by the second snapshot metadata is a same value as asecond value of the second logical storage element specified by thewrite operation; and determining whether to modify the snapshot data andsnapshot metadata of the second snapshot based at least in part onwhether the first value is the same value as the second value.Determining whether the first outstanding write request requiresmodification of snapshot data of the second snapshot may include:determining that the first snapshot metadata does not reference thefirst current value of the first logical storage element; determiningthat the second snapshot metadata does not reference the second currentvalue of the second logical storage element; storing the second value ofthe second logical storage element at a location on the second storagesystem; and modifying the snapshot metadata of the second snapshot toreference the location.

In another embodiment of the invention, as system includes: a firststorage system; a second storage system; a third storage system; alogical storage unit synchronously replicated between the first storagesystem and the second system, wherein the logical storage unit isasynchronously replicated from the first storage system to the thirdstorage system and asynchronously replicated from the second storagesystem to the third storage system; and executable logic that implementsa method. The method includes: at a first point in time, suspendinginitiating of processing on the first storage system of new writerequests for the first logical storage unit received from the hostsystem after the first point in time, recording at least a firstoutstanding write request for the first logical storage unit on thefirst storage system that is outstanding at the first point in time,activating the first snapshot on the first storage system, initiatingactivation on the second storage system of a second snapshot of thesecond logical storage unit as a remote replica of the first snapshot;and, after the recording, the activating and the initiating: resumingthe initiating of processing of new write requests for the first logicalstorage unit on the first storage system, and applying the at leastfirst outstanding write request to the second snapshot. The applying maybe performed as part of a process executing independently of, andconcurrently with, one or more processes that are remotely replicatingwrite operations of the first logical storage unit to the second logicalstorage unit. Applying the at least first outstanding write request mayinclude, for each outstanding write request of the at least firstoutstanding write request: determining whether the outstanding writerequest requires modification of snapshot data of the second snapshot;and, if it is determined that the outstanding write request requiresmodification of the snapshot, modifying the snapshot data and snapshotmetadata of the second snapshot on the second storage system based onthe outstanding write request. The first outstanding write request mayspecify a write operation for a first logical storage elementcorresponding to a second logical storage element of the second logicalstorage unit, and determining whether the first outstanding writerequest requires modification of snapshot data of the second snapshotmay include: determining whether first snapshot metadata of the firstsnapshot references a first current value of the first logical storageelement on the first storage system; determining whether second snapshotmetadata of the second snapshot references a second current value of thesecond logical storage element on the second storage system; anddetermining whether to modify the snapshot data and snapshot metadata ofthe second snapshot based at least in part on whether the first snapshotmetadata references the first current value and whether the secondsnapshot metadata references the second current value. Determiningwhether the first outstanding write request requires modification ofsnapshot data of the second snapshot may include: determining that thefirst snapshot metadata references the first current value of the firstlogical storage element; determining that the second snapshot metadatadoes not reference the second current value of the second logicalstorage element; and modifying the snapshot metadata of the secondsnapshot to reference the second current value of the second logicalstorage element. Determining whether the first outstanding write requestrequires modification of snapshot data of the second snapshot mayinclude: determining whether a first value of the second logical storageelement specified by the second snapshot metadata is a same value as asecond value of the second logical storage element specified by thewrite operation; and determining whether to modify the snapshot data andsnapshot metadata of the second snapshot based at least in part onwhether the first value is the same value as the second value.Determining whether the first outstanding write request requiresmodification of snapshot data of the second snapshot may include:determining that the first snapshot metadata does not reference thefirst current value of the first logical storage element; determiningthat the second snapshot metadata does not reference the second currentvalue of the second logical storage element; storing the second value ofthe second logical storage element at a location on the second storagesystem; and modifying the snapshot metadata of the second snapshot toreference the location.

In another embodiment of the invention, for a system including a hostsystem, a first storage system, a second storage system, a first logicalstorage unit for which data is replicated from the first storage systemto a second logical storage unit of the second storage system,computer-readable media having software thereon defining a method ofremotely replicating a first snapshot of the first logical storage unitfrom the first storage system to the second storage system is provided.The software includes: executable code that controls suspending, at afirst point in time, initiating of processing on the first storagesystem of new write requests for the first logical storage unit receivedfrom the host system after the first point in time; executable code thatcontrols recording at least a first outstanding write request for thefirst logical storage unit on the first storage system that isoutstanding at the first point in time; executable code that controlsactivating the first snapshot on the first storage system; executablecode that controls initiating activation on the second storage system ofa second snapshot of the second logical storage unit as a remote replicaof the first snapshot; executable code that controls, after therecording, the activating and the initiating: resuming the initiating ofprocessing of new write requests for the first logical storage unit onthe first storage system, and applying the at least first outstandingwrite request to the second snapshot. Applying the at least firstoutstanding write request may include, for each outstanding writerequest of the at least first outstanding write request: determiningwhether the outstanding write request requires modification of snapshotdata of the second snapshot; and, if it is determined that theoutstanding write request requires modification of the snapshot,modifying the snapshot data and snapshot metadata of the second snapshoton the second storage system based on the outstanding write request. Thefirst outstanding write request may specify a write operation for afirst logical storage element corresponding to a second logical storageelement of the second logical storage unit, and determining whether thefirst outstanding write request requires modification of snapshot dataof the second snapshot may include: determining whether first snapshotmetadata of the first snapshot references a first current value of thefirst logical storage element on the first storage system; determiningwhether second snapshot metadata of the second snapshot references asecond current value of the second logical storage element on the secondstorage system; and determining whether to modify the snapshot data andsnapshot metadata of the second snapshot based at least in part onwhether the first snapshot metadata references the first current valueand whether the second snapshot metadata references the second currentvalue. Determining whether the first outstanding write request requiresmodification of snapshot data of the second snapshot may include:determining that the first snapshot metadata references the firstcurrent value of the first logical storage element; determining that thesecond snapshot metadata does not reference the second current value ofthe second logical storage element; and modifying the snapshot metadataof the second snapshot to reference the second current value of thesecond logical storage element. Determining whether the firstoutstanding write request requires modification of snapshot data of thesecond snapshot may include: determining whether a first value of thesecond logical storage element specified by the second snapshot metadatais a same value as a second value of the second logical storage elementspecified by the write operation; and determining whether to modify thesnapshot data and snapshot metadata of the second snapshot based atleast in part on whether the first value is the same value as the secondvalue. Determining whether the first outstanding write request requiresmodification of snapshot data of the second snapshot may include:determining that the first snapshot metadata does not reference thefirst current value of the first logical storage element; determiningthat the second snapshot metadata does not reference the second currentvalue of the second logical storage element; storing the second value ofthe second logical storage element at a location on the second storagesystem; and modifying the snapshot metadata of the second snapshot toreference the location.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the present invention will become moreapparent from the following detailed description of illustrativeembodiments thereof taken in conjunction with the accompanying drawingsin which:

FIG. 1 is a block diagram illustrating an example of a data storagenetwork, according to embodiments of the invention;

FIG. 2 is a block diagram illustrating an example of a storage systemincluding multiple physically discrete storage processing nodes,according to embodiments of the invention;

FIG. 3 is a block diagram illustrating an example of tables definingrelationships between logical storage units and physical storage deviceson a data storage system, according to embodiments of the invention;

FIG. 4 a block diagram illustrating an example of a table used for athin logical device, according to embodiments of the invention;

FIG. 5 is a block diagram illustrating an example of a data structurefor mapping logical storage unit tracks to cache slots, according toembodiments of the invention;

FIGS. 6A-6C are examples of various embodiments of components configuredfor replication, according to embodiments of the invention;

FIG. 7A is a diagram illustrating an example of a replication datapointer table, according to embodiments of the invention;

FIG. 7B is a diagram illustrating an example of a replication datapointer tree, according to embodiments of the invention;

FIG. 8 is a diagram illustrating an example of a data pool, according toembodiments of the invention;

FIG. 9 is a diagram illustrating an example of a snapshot table,according to embodiments of the invention;

FIG. 10 is a diagram s illustrating an example of a sequence numberpointer table, according to embodiments of the invention;

FIG. 11 is a flow diagram illustrating processing performed inconnection with initiating a targetless snapshot, according toembodiments of the invention;

FIG. 12 is a flow diagram illustrating processing performed inconnection with a write to a logical device after initiating atargetless snapshot, according to embodiments of the invention;

FIG. 13 is a flow diagram illustrating processing performed inconnection with a read operation after initiating a targetless snapshot,according to embodiments of the invention;

FIG. 14 is a flow diagram illustrating an example of a method of loggingwrite operations for remote replication of a snapshot, according toembodiments of the invention;

FIG. 15 is a block diagram illustrating an example of a data structurefor logging write operations for remote replication of a snapshot,according to embodiments of the invention;

FIG. 16 is a flow diagram illustrating an example of a method performedby a first storage system as part of remotely replicating snapshots fromthe first storage system to a second storage system, according toembodiments of the invention; and

FIG. 17 is a flow diagram illustrating an example of a method performedby a second storage system as part of remotely replicating snapshotsfrom a first storage system to the second storage system, according toembodiments of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

On some data storage networks, remote data replication is employedbetween two or more storage systems, where LSUs from each storage systemare logically paired so that the data of an LSU (R1) on one storagesystem (A1), which may be referred to herein a “primary storage system,”is remotely replicated to another LSU (R2, e.g., a replica LSU) on theother storage system (A2), which may be referred to herein as asecondary storage system. This remote replication may be performedsynchronously (synchronous replication) or asynchronously (asynchronousreplication), as described in more detail elsewhere herein.

On some storage systems today, local replication (i.e., to anotherlocation on a same storage system) of an LSU may be performed usingsnapshots, where a snapshot defines a point-in-time image of an LSU;i.e., the state of an LSU at the point in time. There are several knowntechniques for implementing snapshots, including those described in U.S.Pat. No. 7,340,489 to Vishlitzky, et al. titled “Virtual StorageDevices,” issued Mar. 4, 2008, U.S. Pat. No. 9,965,216 to Jaganathan etal., titled “Targetless Snapshots,” issued May 8, 2018 (“Jaganathan”),and U.S. patent application Ser. No. 16/885,702 to Tobin et al., titled“Snapshot Metadata Deduplication,” filed May 28, 2020 (“Tobin”), theentire contents of each of which is hereby incorporated by reference.

It may be desirable to remotely replicate snapshots from one storagesystem (A1) to another storage system (A2). In some embodiments of theinvention, remote replication of snapshots may be managed manually, forexample, via a host system. For example, when targetless snapshots areemployed, a user may manually link an LSU (a target LSU) to anoutstanding snapshot of R1 on A1, e.g., as described in Jaganathan orTobin. The target LSU may be configured for standard replication (e.g.,Dell EMC™ SRDF® as described in more detail herein) such that the targetLSU is replicated to A2. The user also may configure A2 to create asnapshot of the target LSU once it is fully replicated to A2, therebyproducing a snapshot on A2 that is a replica of the outstanding snapshotof R1 to which the target LSU was linked. However, the foregoingapproach may require significant manual effort, and the consumption ofresources that could otherwise be engaged in the remote replication ofwrite operations from R1 to R2, as opposed to snapshot replication. Suchconsumption of resources may result in disruption or performancedegradation of remote replication, especially if snapshots are createdfrequently and/or the LSUs involved have high levels of write activity,which increases the amount of snapshot data that needs to be transferredand snapshot metadata that needs to be updated.

What may be desirable is a way to remotely replicate snapshots that doesnot have the potential drawbacks described above.

Described herein are techniques and mechanisms for remotely replicatingsnapshots from a first LSU (R1) on a first storage system (A1) to asecond replica LSU (R2) on a second storage system (A2), for example,concurrently to remotely replicating (e.g., synchronously) writeoperations for R1 to R2. A process, P, on A1 executing the replicationof the snapshots from R1 to R2 may be a separate process than the one ormore processes on A1 executing remote replication of write operationsfor R1 to R2. In some embodiments, the process P may be given lowpriority on A1 so as to not impair performance of other operations(including remote replication of write operations on A1. For example,the process P may be run as a background process on A1.

In the following description, a snapshot of R1 may be referred to hereinas SSn₁, where n is an ID (e.g., number) of the snapshot, and the number“1” indicates that the snapshot is of R1. A snapshot of R2 that is areplica of a snapshot of R1 may be referred to herein as SSn₂, where nis an ID (e.g., number) of the snapshot being replicated, and the number“2” indicates that the snapshot is of R2. For example, a first snapshotof R1 may be referred to herein as SS1 ₁, and a replica snapshot of thefirst snapshot, a snapshot of R2, may be referred to herein as SS1 ₂;and a second snapshot of R1 may be referred to herein as SS2 ₁, and areplica snapshot of the second snapshot, a snapshot of R2, may bereferred to herein as SS2 ₂, and so on.

In some embodiments, during a first period of time (referred to hereinas a “consistency window”), outstanding write operations for R1 may berecorded, and a pair of snapshots, SS1 ₁ and SS1 ₂ may be activated onR1 and R2, respectively. SS1 ₂ may be updated based on the writeoperations during a second period of time after the consistency windowhas closed. For example, at a first point in time (T1, marking thebeginning of a consistency window), the initiating of processing on A1of new write requests for R1 received from any host system after thefirst point in time may be suspended, and any write request for thefirst LSU that is outstanding at the first point in time may be recorded(e.g., logged in a data structure) on A1. As used herein, an“outstanding I/O request” is an I/O request for which processing hasbeen initiated on a storage system but for which the storage system hasnot yet acknowledged to the host system that the storage system hascompleted the I/O request. During the consistency window, SS1 ₁ may betaken (i.e., activated) on A1, and SS1 ₂ of R2 may be taken. At leastinitially, SS1 ₂ may not be an exact replica of SS1 ₁ given that SS1 ₂may have been taken before write operations of R1 have been replicatedto R2. The actions of suspending initiating of write operation,recording outstanding write operations and activating SS1 ₁ and SS1 ₂may be considered part of a first phase (Phase 1) of remotelyreplicating snapshots.

As used herein, “taking a snapshot” (or synonymously “activating asnapshot”) means updating snapshot metadata for an LSU so that a latestsnapshot reflects the state of an LSU (e.g., R1) at the current point intime (i.e., the point in time the snapshot is taken). It should beappreciated that a snapshot may be created prior to beingtaken/activated in that that basic parameters (e.g., an ID, sequencenumber) of the snapshot may be defined, for example, as described inJaganathan, before the snapshot is taken.

After a time, T2, marking the end of the consistency window, theinitiating of processing of new write requests for R1 on A1 may beresumed, and the one or more outstanding write requests applied to SS1₂. The outstanding write requests may be applied concurrently to theongoing replication of write requests from R1 to R2, for example as partof a separate (e.g., background) process. Applying the outstanding writerequests may be considered a second phase (Phase II) of remotelyreplicating snapshots.

Applying the outstanding write requests for SS1 ₁ in Phase 2 may includeprocessing on A1 and A2. The A1 processing may include accessing a datastructure on A1 in which outstanding write requests are logged. In someembodiments, the log entries may be indexed according to the logicalstorage element (LSE) of R1 to which they apply. LSEs, for example,tracks, sub-portions thereof, or other types of elements or portionsthereof, are described in more detail elsewhere herein. The SS1 ₁metadata of an LSE for R1 (“R1 LSE”) may be in one of two general R1 LSEstates when processing begins on the R1 LSE during Phase 2: 1) pointing(directly or indirectly) to the current data of the R1 LSE; or 2)pointing (directly or indirectly) to snapshot data of the R1 LSE. Itshould be appreciated that, as used herein, a thing (e.g., metadata)that “references” or “points to” another thing (e.g., data), without aqualifier of “directly” or “indirectly,” may either directly orindirectly reference or point to the thing. For example, a metadatavalue that points to or references data may do so directly by specifyingthe storage location itself of the data, or may do so indirectly byreferencing or pointing to other metadata (e.g., in another datastructure) that may specify the storage location itself of the data orreference/point to other metadata (and so on) that ultimately specifiesthe storage location itself of the data.

As used herein, “snapshot data” is data that is persisted (e.g., stored)exclusively for use by snapshots to reflect the state of LSUs at thedifferent points in time represented by the snapshots. For example, insome snapshot technologies, such as those described in Jaganathan andTobin, current LSU data may be shared by a snapshot of the LSUinitially, at the time at which the snapshot is taken. That is, after asnapshot is taken, but before any write operations to any LSEs of theLSU following the snapshot being taken, the snapshot metadata points tothe same data pointed to by the LSU metadata representing the currentstate of the LSU. It is only after a first write to an LSE of the LSUfollowing the taking of the snapshot that the snapshot data and the LSUdata may diverge, resulting in the moving of the old data for the LSE toa new location (e.g., from a pool of storage reserved for snapshots),and an updating of the snapshot metadata to point to the data at the newlocation. This old data pointed-to by the snapshot metadata is nowsnapshot data that is exclusively used for snapshots, as opposed to thenew data of the write operation, the location of which is now pointed toby current LSU metadata for the LSE, and also may be shared by futuresnapshots. Examples of data structures for storing LSU metadata aredescribed in more detail herein.

During Phase 2, A1 may determine, for each R1 LSE having a loggedoutstanding write request, whether the R1 LSE metadata points to thecurrent data of the R1 LSE, or points to snapshot data of the R1 LSE. Ifit is determined that the R1 LSE metadata points to the current data ofthe R1 LSE, then it may be determined whether the remote replication ofany write operations for the R1 LSE are still pending. If so, Phase 2processing of any outstanding write requests of R1, or perhaps only theLSE in particular, may be suspended until acknowledgements are receivedfrom A2 that the outstanding write requests for the R1 LSE have beencompleted (e.g., cached or stored on a physical storage device) on A2.In some embodiments, rather than waiting for the replication-pendingwrites of the LSE to complete, it may be may recorded (e.g., in a datastructure) that the R1 LSE has replication-pending writes, and the Phase2 processing of other R1 LSEs on A1 may continue. A list of R1 LSEshaving replication-pending writes may be maintained on A1. After Phase 2processing for the last R1 LSE not having replication-pending writes iscompleted, the Phase 2 processing may return to the R1 LSEs havingreplication-pending writes, for example, by accessing the list ofreplication-pending LSEs. Returning to the list of R1 LSEs havingreplication-pending writes may be done in consideration that thereplication-pending write operations of the LSEs may have beenreplicated to R2 (as part of typical replication processing) during thetime that has elapsed since last checked, so that the previously notedreplication-pending writes have now been acknowledged by A2 as completeon A2. For any R1 LSE that still has a replication-pending write, theprocess may move on to the next R1 LSE of the list and attempt furtherPhase 2 processing of the R1 LSE, and repeat the foregoing process untilthere are no longer any replication-pending LSEs.

For each R1 LSE having an outstanding write request, the outstandingwrite operation of the request may be communicated to A2 if: a) it wasdetermined that the SS1 ₁ metadata of the R1 LSE points to snapshot dataof the R1 LSE; orb) it was determined that the R1 LSE metadata points tothe current data of the R1 LSE and either: i) it is determined thatthere are no replication-pending write requests for the R1 LSE, or ii)after acknowledgements are received that the replication of any suchwrite requests are complete on A2. The communication of the outstandingwrite operation may be referred to herein as an outstanding writeinstruction (“OWI”)) from A1 to A2, and may specify any of: an ID of SS1₁; an ID of the R1 LSE; an ID of R2; and an indication of whether thevalue of the R1 LSE metadata for SS1 ₁ is the current value of R1 LSE,i.e., points to the storage location of the current value of R1 LSE, oris a value of snapshot data for R1 LSE for SS1 ₁, i.e., points to alocation of snapshot data specific to R1 LSE for SS1 ₁.

The SS1 ₂ metadata of an LSE of R2 corresponding to the R2 LSE may be inone of three general R2 LSE states when Phase 2 begins: a) pointing tothe current data of the R2 LSE; or b) pointing to snapshot data of theR2 LSE, which is the same as snapshot data of the R1 LSE sent by A1; orc) pointing to snapshot data of the R2 LSE, which is different thansnapshot data of the R1 LSE sent by A1 (and thus incorrect). The storagesystem A2 may determine: the R1 LSE state from the OWI sent by A1, theR2 LSE state, and whether to modify SS1 ₂ snapshot data and snapshotmetadata based at least in part on the R1 LSE state and the R2 LSEstate, as described in more detail elsewhere herein.

While embodiments herein discuss the remote replication of snapshotsfrom a single LSU, R1 to another LSU R2, it should be appreciated thatthe invention is not so limited, as the techniques and mechanismsdescribed herein may be applied to remotely replicating snapshots ofmultiple LSUs on A1 to multiple LSUs on A2 and or other secondary ortertiary storage systems. Further, while embodiments herein discuss theremote replication of a single snapshot from R1 to R2, it should beappreciated that the invention is not so limited, as the techniques andmechanisms described herein may be applied to remotely replicatingmultiple snapshots of R1 to R2, including multiple snapshots takenduring a same consistency window on A1.

Illustrative embodiments of the invention will now be described in moredetail in relation to the figures.

FIG. 1 illustrates an example of an embodiment of a data storage network10 (often referred to herein as a “storage network”). The storagenetwork 10 may include any of: host systems (i.e., “hosts”) 14 a-n;network 18; one or more storage systems 20 a-n; other components; or anysuitable combination of the foregoing. Storage systems 20 a-n, connectedto host systems 14 a-n through network 18, may collectively constitute adistributed storage system 20. All of the host computers 14 a-n andstorage systems 20 a-n may be located at the same physical site, or,alternatively, two or more host computers 14 a-n and/or storage systems20 a-n may be located at different physical locations. Storage network10 or portions thereof (e.g., one or more storage systems 20 a-n incombination with network 18) may be any of a variety of types of storagenetworks, such as, for example, a storage area network (SAN), e.g., of adata center.

Embodiments of the invention are described herein in reference tostorage system 20 a, but it should be appreciated that such embodimentsmay be implemented using other discrete storage systems (e.g., storagesystem 20 n), alone or in combination with storage system 20 a.

The N hosts 14 a-n may access the storage system 20 a, for example, inperforming input/output (I/O) operations or data requests, throughnetwork 18. For example, each of hosts 14 a-n may include one or morehost bus adapters (HBAs) (not shown) that each include one or more hostports for connecting to network 18. The network 18 may include any oneor more of a variety of communication media, switches and othercomponents known to those skilled in the art, including, for example: arepeater, a multiplexer or even a satellite. Each communication mediummay be any of a variety of communication media including, but notlimited to: a bus, an optical fiber, a wire and/or other type of datalink, known in the art. The network 18 may include at least a portion ofthe Internet, or a proprietary intranet, and components of the network18 or components connected thereto may be configured to communicate inaccordance with any of a plurality of technologies, including, forexample: SCSI, ESCON, Fibre Channel (FC), iSCSI, FCoE, GIGE (GigabitEthernet), NVMe over Fabric (NVMeoF); other technologies, or anysuitable combinations of the foregoing, each of which may have one ormore associated standard specifications. In some embodiments, thenetwork 18 may be, or include, a switching fabric including one or moreswitches and other components. A network located externally to a storagesystem that connects host systems to storage system resources of thestorage system, may be referred to herein as an “external network.”

Each of the host systems 14 a-n and the storage systems 20 a-n includedin the storage network 10 may be connected to the network 18 by any oneof a variety of connections as may be provided and supported inaccordance with the type of network 18. The processors included in thehost computer systems 14 a-n may be any one of a variety of proprietaryor commercially available single or multi-processor system, such as anIntel-based processor, or other type of commercially available processorable to support traffic in accordance with each particular embodimentand application. Each of the host computer systems may perform differenttypes of I/O operations in accordance with different tasks andapplications executing on the hosts. In the embodiment of FIG. 1, anyone of the host computers 14 a-n may issue an I/O request to the storagesystem 20 a to perform an I/O operation. For example, an applicationexecuting on one of the host computers 14 a-n may perform a read orwrite operation resulting in one or more I/O requests being transmittedto the storage system 20 a.

Each of the storage systems 20 a-n may be manufactured by differentvendors and inter-connected (not shown). Additionally, the storagesystems 20 a-n also may be connected to the host systems through any oneor more communication connections 31 that may vary with each particularembodiment and device in accordance with the different protocols used ina particular embodiment. The type of communication connection used mayvary with certain system parameters and requirements, such as thoserelated to bandwidth and throughput required in accordance with a rateof I/O requests as may be issued by each of the host computer systems 14a-n, for example, to the storage systems 20 a-20 n. It should beappreciated that the particulars of the hardware and software includedin each of the components that may be included in the storage systems 20a-n are described herein in more detail, and may vary with eachparticular embodiment.

Each of the storage systems, such as 20 a, may include a plurality ofphysical storage devices 24 (e.g., physical non-volatile storagedevices) such as, for example, disk devices, solid-state storage devices(SSDs, e.g., flash, storage class memory (SCM), NVMe SSD, NVMe SCM) oreven magnetic tape, and may be enclosed within a disk array enclosure(DAE) 27. In some embodiments, two or more of the physical storagedevices 24 may be grouped or arranged together, for example, in anarrangement consisting of N rows of physical storage devices 24 a-n. Insome embodiments, one or more physical storage devices (e.g., one of therows 24 a-n of physical storage devices) may be connected to a back-endadapter (“BE”) (e.g., a director configured to serve as a BE)responsible for the backend management of operations to and from aportion of the physical storage devices 24. A BE is sometimes referredto by those in the art as a disk adapter (“DA”) because of thedevelopment of such adapters during a period in which disks were thedominant type of physical storage device used in storage systems, eventhough such so-called DAs may be configured to manage other types ofphysical storage devices (e.g., SSDs). In the system 20 a, a single BE,such as 23 a, may be responsible for the management of one or more(e.g., a row) of physical storage devices, such as row 24 a. That is, insome configurations, all I/O communications with one or more physicalstorage devices 24 may be controlled by a specific BE. BEs 23 a-n mayemploy one or more technologies in communicating with, and transferringdata to/from, physical storage devices 24, for example, SAS, SATA orNVMe. For NVMe, to enable communication between each BE and the physicalstorage devices that it controls, the storage system may include a PCIeswitch for each physical storage device controlled by the BE; i.e.,connecting the physical storage device to the controlling BE.

It should be appreciated that the physical storage devices are notlimited to being arranged in rows. Further, the DAE 27 is not limited toenclosing disks, as the name may suggest, but may be constructed andarranged to enclose a plurality of any type of physical storage device,including any of those described herein, or combinations thereof.

The system 20 a also may include one or more front-end adapters (“FAs”)21 a-n (e.g., directors configured to serve as FAs), which also arereferred to herein as host adapters (“HAs”). Each of these FAs may beused to manage communications and data operations between one or morehost systems and global memory (GM) 25 b of memory 26. The FA may be, orinclude, a Fibre Channel (FC) adapter if FC is a technology being usedto communicate between the storage system 20 a and the one or more hostsystems 14 a-n, or may be another type of adapter based on the one ormore technologies being used for I/O communications.

Also shown in the storage system 20 a is a remote adapter (“RA”) 40. TheRA may be, or include, hardware that includes a processor used tofacilitate communication between storage systems (e.g., 20 a and 20 n),such as between two of the same or different types of storage systems,and/or may be implemented using a director.

Storage system 20 a also may include a management module 22, which maybe configured (e.g., dedicated) to performing storage managementfunctions or services such as, for example, storage provisioning, deviceconfiguration, tier management, other services, or any combination ofother services. The management module may be configured to be accessedby only certain personnel (e.g., storage administrators, supportengineers) and may have its own dedicated hardware, firmware, software,CPU resources and OS, and may be loaded with one or more applications,tools, CLIs, APIs and the like to enable management. In someembodiments, the management module, or portions thereof, may be locatedexternal to storage system 20 a, for example, as part of one of hostsystems 14 a-n or another separate system connected to storage system 20a via network 18.

The FAs, BEs and RA may be collectively referred to herein as directors37 a-n. Each director 37 a-n may be implemented (e.g., in hardware,firmware, software or a combination thereof) on a circuit board thatincludes memory resources (e.g., at least a segment of GM portion 25 b)and compute resources, for example, one or more processing cores (e.g.,as part of a CPU) and/or a CPU complex for processing I/O operations,and that as described in more detail elsewhere herein. There may be anynumber of directors 37 a-n, which may be limited based on any of anumber of factors, including spatial, computation and storagelimitations. In an embodiment disclosed herein, there may be up tosixteen directors coupled to the memory 26. Other embodiments may use ahigher or lower maximum number of directors.

System 20 a also may include an internal switching fabric (i.e.,internal fabric) 30, which may include one or more switches, thatenables internal communications between components of the storage system20 a, for example, directors 37 a-n (FAs 21 a-n, BEs 23 a-n, RA 40,management module 22) and memory 26, e.g., to perform I/O operations.One or more internal logical communication paths may exist between thedirectors and the memory 26, for example, over the internal fabric 30.For example, any of the directors 37 a-n may use the internal fabric 30to communicate with other directors to access any of physical storagedevices 24; i.e., without having to use memory 26. In addition, one ofthe directors 37 a-n may be able to broadcast a message to all of theother directors 37 a-n over the internal fabric 30 at the same time.Each of the components of system 20 a may be configured to communicateover internal fabric 30 in accordance with one or more technologies suchas, for example, InfiniBand (TB), Ethernet, Gen-Z, another technology,or any suitable combination of the foregoing.

The GM portion 25 b may be used to facilitate data transfers and othercommunications between the directors 37 a-n in a storage system. In oneembodiment, the directors 37 a-n (e.g., serving as FAs or BEs) mayperform data operations using a cache 28 that may be included in the GM25 b, for example, in communications with other directors, and othercomponents of the system 20 a. The other portion 25 a is that portion ofmemory that may be used in connection with other designations that mayvary in accordance with each embodiment. Global memory 25 b and cache 28are described in more detail elsewhere herein. It should be appreciatedthat, although memory 26 is illustrated in FIG. 1 as being a single,discrete component of storage system 20 a, the invention is not solimited. In some embodiments, memory 26, or the GM 25 b or other memory25 a thereof, may be distributed among a plurality of physicallydiscrete processing nodes (e.g., circuit boards) as described in moredetail elsewhere herein.

In at least one embodiment, write data received at the storage systemfrom a host or other client may be initially written to cache 28 andmarked as write pending. For example, cache 28 may be partitioned intoone or more portions called cache slots (which also may be referred toin the field of data storage as cache lines, cache blocks or anothername), which may be a of a predefined uniform size, for example, 128Kbytes. Write data of a write operation received at the storage systemmay be initially written (i.e., staged) in one or more of these cacheslots and marked as write pending. Once written to cache 28, the host(e.g., one of 14 a-n) may be notified that the write operation hascompleted. At a later time, the write data may be de-staged from cache28 to one or more physical storage devices 24 a-n, such as by a BE.

The memory 26 may include persistent memory for which for which datastored thereon persists after the process or program that created thedata terminates. For example, at least portions of the memory 26 may beimplemented using DIMM (or another type of fast RAM memory) that isbattery-backed by a NAND-type memory (e.g., flash). In some embodiments,the data in such persistent memory may persist (for at least some periodof time) after the storage system fails. The memory 26 (or at least aportion thereof—e.g., the cache 28 or a portion thereof) may beconfigured such that each data written to the memory 28 is mirrored toprovide a form of write protection. For example, each memory locationwithin each such mirrored portion of the memory 26 may have acorresponding memory location on the storage system 20 a to which aredundant copy of the data is stored, and which can be used in place ofthe mirrored memory location in the event the mirrored memory locationfails. The redundant memory location should be located outside of atleast the most local fault zone of the mirrored memory location. In someembodiments described in more detail herein, the memory 26 may bedistributed among multiple physically discrete processing nodes (e.g.,circuit boards), in which case mirroring may be configured such that amirrored memory location and its corresponding redundant memory locationare located on different physically discrete processing nodes.

Storage system 20 a may include a back-up power supply 41 (e.g., abattery) that can provide power to the storage system for a limitedamount of time to after primary (AC) power fails. This limited time mayallow certain tasks to be performed during a window of time beginningwhen the primary power fails until the earliest of: the primary power isrestored; and the end of the limited lifetime (sometimes on the order ofsecond or tens of seconds) of the back-up power supply. For example, thestorage system 20 a (e.g., the memory 26 and/or memory management module32) may be configured to automatically copy the contents of the memory26 during this window of time to one or more predetermined physicalstorage devices, to be restored to the memory 26 after the power hasbeen restored, e.g., as part of the storage system recovering process.Such automatic copying for restoration during recovering may referred toherein as “vaulting.” Vaulting may provide a form of write protectionfor data written to the memory 26, for example, for dirty data in thecache 28; i.e., data written to the storage system, which has beenstaged in the cache 28 but not yet de-staged to a physical storagedevice. More broadly, vaulting may be performed for any data written tothe memory 26.

The storage system 20 a may include a memory management module 32configured to manage one or more aspects of the memory 26, and thememory management module 32 may include a cache management module 34 formanaging one or more aspects of the cache 28.

It should be noted that, although examples of techniques herein may bemade with respect to a physical storage system and its physicalcomponents (e.g., physical hardware for each RA, BE, FA and the like),techniques herein may be performed in a physical storage systemincluding one or more emulated or virtualized components (e.g., emulatedor virtualized ports, emulated or virtualized BEs or FAs), and also avirtualized or emulated storage system including virtualized or emulatedcomponents. For example, in embodiments in which NVMe technology is usedto communicate with, and transfer data between, a host system and one ormore FAs, one or more of the FAs may be implemented using NVMetechnology as an emulation of an FC adapter.

Any of storage systems 20 a-n, or one or more components thereof,described in relation to FIGS. 1-2 may be implemented using one or moreSymmetrix™, VIVIAX™, VIVIAX3™ or PowerMax™ systems made available fromDell EMC.

Host systems 14 a-n may provide data and control (e.g., management andaccess control) information to storage systems 20 a-n over a pluralityof I/O paths defined between the host systems and storage systems, forexample, including host system components, storage system components,and network components (e.g., of network 18), and the storage systemsalso may provide data to the host systems across the I/O paths. In theembodiment of FIG. 1, the host systems may not address the physicalstorage devices (e.g., disk drives or flash drives) 24 of the storagesystems directly, but rather access to data may be provided to one ormore host systems from what the host systems view as a plurality of LSUsincluding, for example, logical blocks, logical devices (also referredto as logical volumes, LUNs, logical storage units and/or logicaldisks), thin devices, groups of logical devices (e.g., storage groups),NVMe namespaces, and other types of LSUs. For example, a PowerMaxstorage system may be configured to organize available storage resources(e.g., physical storage devices) into many LUNs, each with its ownaddressable space defined in logical blocks addresses (LBAs). The LSUsmay or may not correspond to the actual physical storage devices. Forexample, one or more LSUs may map to a single physical storage device;that is, the logical address space of the one or more LSU may map tophysical space on a single physical storage device. Data in a singlestorage system may be accessed by multiple hosts allowing the hosts toshare the data residing therein. The FAs may be used in connection withcommunications between a storage system and a host system. The RAs maybe used in facilitating communications between two storage systems. TheBEs may be used in connection with facilitating communications to theassociated physical storage device(s) based on LSU(s) mapped thereto.

FIG. 2 is a block diagram illustrating an example of at least a portion211 of a storage system (e.g., 20 a) including multiple, physicallydiscrete storage processing nodes (e.g., circuit boards) 212 a-212 n,which may be referred to herein as “processing nodes.” Storage system211 may include a plurality of processing nodes 212 a-212 n and a fabric230 (e.g., internal fabric 30) over which the processing nodes 212 a-nmay communicate. Each of the processing nodes 212 a-212 n may includecomponents thereon as illustrated. The switching fabric 230 may include,for example, one or more switches and connections between the switch(es)and processing nodes 212 a-212 n. In at least one embodiment, the fabric230 may be an IB fabric. In some embodiments, multiple processing 212a-n nodes may be implemented on a single physically discrete component;e.g., two processing nodes 212 a-n may be implemented on single engineof PowerMax storage system.

In the following paragraphs, further details are described withreference to processing node 212 a but each of the N processing nodes ina system may be similarly configured. For example, processing node 212 amay include any of: one or more directors 216 a (e.g., directors 37a-n); memory portion 214 a; one or more processing cores 217 a includingcompute resources, for example, as part of a CPUs and/or a CPU complexfor processing I/O operations; and a fabric interface module (FIM) 215 afor interfacing the processing node 212 a to an internal fabric 230.Each director 216 a may be configured to operate, such as by executingcode, as any one or more of an FA, BE, RA, and the like. In someembodiments, each of the directors, or a portion thereof, areimplemented in software stored in a memory portion 214 a (e.g., in adedicated local memory 222 a) that is executed by one or more of theprocessing cores 217 a. Such software implementation of directors may beconsidered emulations of types of physical directors (i.e., directorsimplemented (at least primarily) in hardware).

Each FIM 215 a-n may include one or more host channel adapters (HCAs)that physically couple, and are configured to enable communicationbetween, its respective processing node 212 a-n, and the internal fabric230. In some embodiments, the internal fabric 230 may include multiple(e.g., 2) switches, and each HCA 215 a-n may have multiple (e.g., 2)ports, each one connected directly to one of the switches.

Each of the processing nodes 212 a-n may, respectively, also includememory portions 214 a-n. The memory portion of each processing node maybe characterized as locally accessible with respect to that particularprocessing node, and more specifically with respect to other componentson the same processing node. For example, processing node 212 a includesmemory portion 214 a which is memory that is local to that particularprocessing node 212 a. Data stored in memory portion 214 a may bedirectly accessed by any of the processing cores 217 a (e.g., executinginstructions on behalf of one of the directors 216 a) of the processingnode 212 a. For example, memory portion 214 a may be a fast memory(e.g., DIMM (dual inline memory module) DRAM (dynamic random accessmemory)) that is locally accessible by a director 216 a, where data fromone location in 214 a may be copied to another location in 214 adirectly using DMA operations (e.g., local memory copy operations)issued by director 216 a. Thus, the director 216 a may directly accessdata of 214 a locally without communicating over the fabric 230.

The memory portions 214 a-214 n of processing nodes 212 a-n may befurther partitioned into different portions or segments for differentuses. For example, each of the memory portions 214 a-214 n mayrespectively include GM segments 220 a-n configured for collective useas segments of a distributed GM, for example, GM 225 (e.g., GM 25 b).Thus, data stored in any GM segment 220 a-n may be accessed by anydirector 216 a-n on any processing node 212 a-n. Additionally, each ofthe memory portions 214 a-n may respectively include dedicated localmemories 222 a-n. Each of the dedicated local memories 222 a-n arerespectively configured for use locally by the one or more directors 216a-n, and possibly other components, residing on the same singleprocessing node. In at least one embodiment where there is a singledirector denoted by 216 a (and generally by each of 216 a-n), datastored in the dedicated local memory 222 a may be accessed by therespective single director 216 a located on the same processing node 212a. However, the remaining directors located on other ones of the Nprocessing nodes may not access data stored in the dedicated localmemory 222 a.

To further illustrate, GM segment 220 a may include information such asuser data stored in the cache portion 220 a, metadata, and the like,that is accessed (e.g., for read and/or write) generally by any directorof any of the processing nodes 212 a-n. Thus, for example, any director216 a-n of any of the processing nodes 212 a-n may communicate over thefabric 230 to access data in GM segment 220 a. In a similar manner, anydirector 216 a-n of any of the processing nodes 212 a-n may generallycommunicate over fabric 230 to access any GM segment 220 a-n of thedistributed GM. Although a particular GM segment, such as 220 a, may belocally accessible to directors on one particular processing node, suchas 212 a, any director of any of the processing nodes 212 a-n maygenerally access the GM segment 220 a. Additionally, the director 216 aalso may use the fabric 230 for data transfers to and/or from GM segment220 a even though 220 a is locally accessible to director 216 a (withouthaving to use the fabric 230).

Also, to further illustrate, dedicated local memory 222 a may be asegment of the memory portion 214 a on processing node 212 a configuredfor local use solely by components on the single/same processing node212 a. For example, dedicated local memory 222 a may include datadescribed in following paragraphs which is used and accessed only bydirectors 216 a included on the same processing node 212 a as thededicated local memory 222 a. In at least one embodiment in accordancewith techniques herein and as described elsewhere herein, each of thededicated local memories 222 a-n may include a local page table or pagedirectory used, respectively, by only director(s) 216 a-n local to eachof the processing nodes 212 a-n.

In such an embodiment as in FIG. 2, the GM segments 220 a-n may belogically concatenated or viewed in the aggregate as forming onecontiguous GM logical address space of a distributed GM. In at least oneembodiment, the distributed GM formed by GM segments 220 a-n may includethe cache portion 254 a, various metadata and/or structures, and otherinformation, as described in more detail elsewhere herein. Consistentwith discussion herein, the cache portion 254 a, having cache slotsallocated from GM segments 220 a-n, may be used to store I/O data (e.g.,for servicing read and write operations).

Each cache portion 254 a-n may be a portion of a shared cache 228 (e.g.,cache 28) distributed across the processing nodes 212 a-n, where theshared cache 228 may be considered a part of the GM 225. The cacheportion 254 a-n may include a plurality of cache slots 256 a-n, eachcache slot including one or more (e.g., 16) sections 258 a-n. Each cacheslot 256 a-n may be of a uniform size (e.g., 128 KB) and each sectionmay be of a uniform size (e.g., 8 KB). It should be appreciated thatcache slot sizes and section sizes other than 128 KB and 8 KB, and aquantity of sections other than 16, may be used.

In an embodiment, the storage system as described may be characterizedas having one or more logical mapping layers in which an LSU of thestorage system is exposed to the host whereby the LSU is mapped by suchmapping layers of the storage system to one or more physical storagedevices. Additionally, the host also may have one or more additionalmapping layers so that, for example, a host-side LSU may be mapped toone or more storage system LSUs as presented to the host.

Any of a variety of data structures may be used to process I/O onstorage system 20 a, including data structures to manage the mapping ofLSUs and locations thereon to physical storage devices and locationsthereon. Such data structures may be stored in any of memory 26,including GM 25 b and memory 25 a, GM segment 220 a-n and/or dedicatedlocal memories 22 a-n. Thus, storage system 20 a, and storage system 620a described in more detail elsewhere herein, may include memory elements(e.g., cache) that hold data stored on physical storage devices or thatis currently held (“staged”) and will be stored (“de-staged”) tophysical storage devices, and memory elements that store metadata (e.g.,any of the metadata described herein) associated with such data.Illustrative examples of data structures for holding such metadata willnow be described.

FIG. 3 is a block diagram illustrating an example of tables 60 definingrelationships between LSUs and physical storage devices on a datastorage system, according to embodiments of the invention. A first table62 corresponds to the LSUs (e.g., logical deices) used by a storagesystem (e.g., storage system 20 a) or by an element of a storage system,such as an FA and/or a BE, and may be referred to herein as a “masterLSU table.” The master LSU table 62 may include a plurality of LSUentries 66-68, each entry representing an LSU used by the storagesystem. The entries in the master LSU table 62 may include descriptionsfor any type of LSU described herein.

Each of the entries 66-68 of the master LSU table 62 may correspond to,and include a reference to, another table corresponding to the LSUrepresented by the respective entry. For example, the entry 67 mayreference a table 72, referred to herein as an “LSU table,”corresponding to the LSU represented by the entry 67. The LSU table 72may include a header that contains information pertinent to the LSU as awhole. The LSU table 72 also may include entries 76-78 for separatecontiguous logical data portions of the represented LSU; each suchlogical data portion corresponding to, and including a reference to, oneor more contiguous physical locations (e.g., logical block addressranges) of a physical storage device (e.g., a cylinder and/or a group oftracks). In an embodiment disclosed herein, an LSU may contain anynumber of logical data portions depending upon how the LSU isinitialized. However, in other embodiments, an LSU may contain a fixednumber of logical data portions.

Each of the logical data portion entries 76-78 may correspond to a tracktable. For example, the entry 77 may correspond to a track table (or“LSU track table”) 82, which includes a header 84. The LSU track table82 also includes entries 86-88, each entry representing an LSU track ofthe entry 77. As used herein, a “track” or “LSU track” represents acontiguous segment of physical storage space on a physical storagedevice. In an embodiment disclosed herein, there are fifteen tracks foreach contiguous logical data portion. However, for other embodiments, itmay be possible to have different numbers of tracks for each of thelogical data portions or even a variable number of tracks for eachlogical data portion. The information in each of the LSU track entries86-88 may include a pointer (either direct or indirect—e.g., throughanother data structure) to a physical address of a physical storagedevice, for example, any of physical storage devices 24 of the storagesystem 20 a (or a remote storage system if the system is so configured).

In addition to physical storage device addresses, or as an alternativethereto, each of the LSU track entries 86-88 may include a pointer(either direct or indirect—e.g., through another data structure) to oneor more cache slots of a cache in the GM if the data of the logicaltrack is currently in cache. For example, an LSU track entry 86-88 maypoint to one or more entries of cache slot table 300, described in moredetail elsewhere herein. Thus, the LSU track table 82 may be used to maplogical addresses of an LSU corresponding to the tables 62, 72, 82 tophysical addresses within physical storage devices of a storage systemand/or to cache slots within a cache. In some embodiments, each entry86-88 may specify a version of the data stored on the track. Asub-element of an LSU, for example, a logical storage portion or track,may be referred to herein as a logical storage element (LSE).

FIG. 4 is a diagram illustrating an example of a table 72′ used for athin logical device (i.e., a thin LSU), which may include null pointersas well as entries similar to entries for the LSU table 72, discussedabove, that point to a plurality of LSU track tables 82 a-82 e. Table72′ may be referred to herein as a “thin device table.” A thin logicaldevice may be allocated by the system to show a particular storagecapacity while having a smaller amount of physical storage that isactually allocated. When a thin logical device is initialized, all (orat least most) of the entries in the thin device table 72′ may be set tonull. Physical data may be allocated for particular sections as data iswritten to the particular logical data portion. If no data is written toa logical data portion, the corresponding entry in the thin device table72′ for the logical data portion maintains the null pointer that waswritten at initialization.

FIG. 5 is a block diagram illustrating an example of a data structure300 for mapping LSU tracks (e.g., thin device tracks) to cache slots ofa cache. Data structure 300 may be referred to herein as a “cache slottable.” The cache slot table 300 may include a plurality of entries(i.e., rows) 302, each row representing an LSU track (e.g., any of LSUtracks 86-88 in track table 82) identified by an LSU ID in column 304and an LSU track ID (e.g., number) identified in column 306. For eachentry of the cache slot table 300, a column 312 may specify (e.g., usinga cache slot ID and/or memory address) a cache location in a cachecorresponding to the logical storage device track specified by columns304 and 306. A combination of an LSU identifier and LSU track identifiermay be used to determine from columns 304 and 306 whether the data ofthe identified LSU track currently resides in any cache slot identifiedin column 312. Through use of information from any of tables 62, 72, 72′and 82 described in more detail elsewhere herein, the one or more LSUtracks of an LSU specified in an I/O operation can be mapped to one ormore cache slots. Further, using the same data structures, the one ormore physical address ranges corresponding to the one or more LSU tracksof the LSU may be mapped to one or more cache slots.

Each of the entries 302 of the cache slot table also may specify: cachelock information in a column 314, replication information in a column316, and other cache information in a column 318. The cache lockinformation may indicate whether or not the cache slot represented bythe entry is locked, and if locked, the process ID of the entity thatowns the lock. The entity may be, for example: an FA executing a writeoperation from a host; an RA replicating a write operation from thecache slot to R2, or replicating a write operation from R2 into thecache slot; or a BE de-staging data in the cache to a physical storagedevice or reading data from a PSD into the cache slot. The replicationinformation may specify information relative to replication, forexample, the replication cycle number currently associated with thecache slot, the replication (e.g., RDF) group associated with the cacheslot (i.e., associated with the R1 track currently mapped to the cacheslot, a type of cache slot (e.g., normal or duplicate), and otherinformation. A normal cache slot type may indicate that a cache slot ishandled per normal processing, i.e., when there is not a cache lockconflict resolution involved, for example, as described herein. Aduplicate cache slot type may indicate that a cache slot is a duplicateof a cache slot used to resolve a cache slot lock conflict, which is nothandled in the standard manner, but rather, is handled differently toresolve the cache slot lock, for example, as described herein.

The other cache slot information in the column 318 may includeinformation about the status of writes to one or more portions (e.g.,sectors) of the R1 track corresponding to the cache slot, e.g., whetherthe write is pending or complete. Completing the write may includewriting it to a PSD on A1 (e.g., de-staging it from cache) and receivingacknowledgement from A2 (and perhaps other remote storage systems towhich the LSU in question is being replicated) that the replicated datais committed on A2 (and other remote storage systems if any).

It should be appreciated that the cache slot table 300 may be used forpurposes independent of any LSU tracks mapped thereto. That is, a cacheslot ID or memory address in cache pointer column 312 may be used as akey to access, and modify as necessary, cache metadata about a cacheslot, including any of the information in columns 314, 316 and/or 318.

The tables 62, 72, 72′, 82 and 300 may be stored in the GM 26 of thestorage system 20 a during operation thereof and may otherwise be storedin non-volatile memory (i.e., with the corresponding physical storagedevice). In addition, tables corresponding to LSUs accessed by aparticular host may be stored in local memory of the corresponding oneof the FAs 21 a-n. In addition, RA 40 and/or the BEs 23 a-n also may useand locally store portions of the tables 62, 72, 72′, 82 and 300. Otherdata structures may be stored in any of GM 25 b, memory 25 a, GM segment220 a-n and/or dedicated local memories 22 a-n.

Any of the information contained in any of the data structures 62, 72,72′, 82 and 300, for example, the information included in the LSU tracktable 82 and the cache slot table 300, may be combined in a single datastructure, which may be referred to herein as an LSU track metadatatable. In some embodiments, a cache slot table 300 may be maintainedseparately from an LSU track metadata table. In such embodiments, theentries 302 of the cache slot table 300 may be indexed/keyed by a cacheslot ID and/or memory address in the column 312, may identify the LSUtrack currently mapped to the slot (if any) in columns 304 and 306, mayinclude cache lock info in the column 314, and may include other cacheinfo. In such embodiments, the LSU track table may include: informationabout the LSU track described in relation to the LSU track table 82;replication information described in relation to the column 316; thecache slot (of any) currently mapped to the LSU track; and any otherinformation described in relation to the cache slot table 300.

In some embodiments of the invention, data replication may be employedbetween two or more storage systems on a storage network, which maybefore referred to herein as “remote data replication” to distinguish itfrom “local data replication,” which may be used herein to refer to datareplication performed within a single storage system. Referring back toFIG. 1, the RA (remote adapter) 40 may be configured to facilitatecommunication between data storage systems, such as between two of thesame or different types of data storage systems. In one embodimentdescribed in more detail in following paragraphs and figures, the RAs ofthe different data storage systems may communicate over a GigabitEthernet or Fibre Channel transmission channel supporting messagingtraffic between data storage systems. The RA (e.g., RA 40) may includehardware including a processor used to facilitate communication betweendata storage systems, such as between two data storage systems. The RAmay be used with the Dell EMC™ Symmetrix® Remote Data Facility (SRDF®)products. Dell EMC™ SRDF® is a family of products that facilitates thedata replication from one data storage array to another through aStorage Area Network (SAN) or and IP network. Dell EMC™ SRDF® logicallypairs a device or a group of devices from each array and replicates datafrom one to the other synchronously or asynchronously. Generally, theDell EMC™ SRDF® products are one example of commercially availableproducts that may be used to provide functionality of a remote datafacility (RDF) for use in an embodiment in connection with techniquesherein.

Referring to FIG. 6A, shown is an example of an embodiment of a system2101 that may be used in connection with the techniques describedherein. It should be noted that the embodiment illustrated in FIG. 6Apresents a simplified view of some of the components illustrated in FIG.1, for example, including only some detail of the data storage system 20a for the sake of illustration.

Included in the system 2101 are data storage systems 2102 and 2104 andhosts 2110 a, 2110 b and 1210 c. The data storage systems 2102, 2104 maybe remotely connected and communicate over network 2122, such as theInternet or other private network, and facilitate communications withthe components connected thereto. Hosts 2110 a, 2110 b and 2110 c mayperform operations to data storage system 2102 over connection 2108 a.The hosts 2110 a, 2110 b and 2110 c may be connected to the data storagesystem 2102 through connection 2108 a which may be, for example, networkor other type of communication connection. Although not illustrated, thehosts 2110 a-2110 c also may be directly connected to a network such asthe Internet.

The data storage systems 2102 and 2104 may include one or more LSUs(e.g., logical storage devices). In this example, data storage system2102 includes R1 2124 and data storage system 104 includes R2 2126. LSUsR1 and R2 may be referred to herein simply as “R1” and “R2.” Both of thedata storage systems may include one or more other logical and/orphysical devices. Data storage system 2102 may be characterized as localwith respect to hosts 2110 a, 2110 b and 2110 c. Data storage system 104may be characterized as remote with respect to hosts 2110 a, 2110 b and2110 c. Each of R1 and R2 may be configured as LUNs.

The host 2110 a may issue a command, such as to write data to R1 of datastorage system 2102. In some instances, it may be desirable to copy datafrom the R1 to another second LSU, such as R2, provided in a differentlocation so that if a disaster occurs that renders R1 inoperable, thehost (or another host) may resume operation using the data of R2. Such acapability is provided, for example, by the Dell EMC™ SRDF® products.Communication between LSUs on different data storage systems using DellEMC™ SRDF® is described, for example, in U.S. Pat. Nos. 5,742,792,5,544,347, and 7,054,883, all of which are incorporated by referenceherein. With Dell EMC™ SRDF®, a user may denote a first LSU, such as R1,as a master LSU and a second LSU, such as R2, as a slave LSU. Otherincarnations of Dell EMC™ SRDF® may provide a peer to peer relationshipbetween the local and remote LSUs. In this example, the host 2110 ainteracts directly with the R1 of data storage system 2102, but any datachanges made are automatically provided to the R2 LSU of data storagesystem 2104 using Dell EMC™ SRDF®. In operation, the host 2110 a mayread and write data using the R1 volume in 2102, and Dell EMC™ SRDF® mayhandle the automatic copying and updating of data from R1 to R2 in datastorage system 2104.

As illustrated in connection with other figures herein, data storagesystem 2102 may have one or more RAs included therein to facilitateremote connections to the data storage system 2104. Communicationsbetween storage system 2102 and 2104 may be made over connections 2108b,2108 c to network 2122. Data storage system 2104 may include one ormore RAs for use in receiving the communications from the data storagesystem 2102. The data storage systems may communicate, for example, overGigabit Ethernet connections supporting TCP/IP traffic. The Dell EMC™SRDF® replication functionality may be facilitated with the RAs providedat each of the data storage systems 2102 and 2104. Performing remotedata communications using SRDF® over a TCP/IP network is described inmore detail in U.S. Pat. No. 6,968,369, Nov. 22, 2005, Veprinsky, etal., “Remote Data Facility Over an IP Network,” which is incorporated byreference herein. In connection with Dell EMC™ SRDF®, a single RDF link,connection or path may be between an RA of the system 2102 and an RA ofthe system 2104. As described in more detail below, techniques aredescribed for use in transmitting data over an RDF link, such as I/Otraffic including write data in connection with performing remote datareplication over the RDF link between the systems 2102 and 2104.

An embodiment also may include the concept of a remote data facility(RDF) group in which one or more LSUs (e.g., LUNs) on a data storagesystem are associated with a particular group under the control of asingle RA which services the LSUs included therein. Rather than have asingle R1 LSU and a single R2 LSU, a grouping may be defined so that asource group of LSUs, such as on data storage system 2102, havecorresponding target LSUs of a target group, such as LSUs on datastorage system 2104. Devices in a source group may be mirrored incorresponding LSUs of a target group using Dell EMC™ SRDF®functionality.

Techniques herein may be used with Dell EMC™ SRDF®, or more generallyany RDF, operating in one or more different supported modes. Forexample, such modes may include Dell EMC™ SRDF® operating in synchronousmode, asynchronous mode, or adaptive copy mode. For example, inconnection with Dell EMC™ SRDF®, the host may issue a write to an R1 LSUin a first data storage system and the data change is propagated to theR2 LSU in a second data storage system. As discussed in U.S. Pat. No.5,544,347, Dell EMC™ SRDF® can be operated in either a synchronous modeor an asynchronous mode. When operating in the synchronous mode, thehost does not consider a write I/O operation to be complete until thewrite I/O has been completed on both the first and second data storagesystems. Thus, in synchronous mode, the first or source storage systemwill not provide an indication to the host that the write operation iscommitted or complete until the first storage system receives anacknowledgement from the second data storage system regarding completionor commitment of the write by the second data storage system. Incontrast, in connection with the asynchronous mode, the host receives anacknowledgement from the first data storage system as soon as theinformation is committed to the first data storage system withoutwaiting for an acknowledgement from the second data storage system.

Depending on the physical distance between the data storage systems2102, 2104, it may be desirable to operate in a mode such asasynchronous to avoid host timeouts while the host awaitsacknowledgement regarding completion of a host I/O.

Described in following paragraphs are techniques that may be used inconnection with performing data replication in a synchronous manner suchas Dell EMC™ SRDF® operating in an synchronous mode (Dell EMC™ SRDF®/S).With synchronous mode data replication, a host 2110 a may issue a writeto the R1 LSU 2124. The primary or R1 data storage system 2102 may storethe write data in its cache at a cache location and mark the cachelocation as including write pending (WP) data as mentioned elsewhereherein. The remote data replication facility operating in synchronousmode, such as Dell EMC™ SRDF®/S, may propagate the write data across anestablished RDF link (more generally referred to as a the remotereplication link or link) such as over 2108 b, 2122, and 2108 c, to thesecondary or R2 data storage system 2104 where the write data may bestored in the cache of the system 2104 at a cache location that ismarked as WP. Once the write data is stored in the cache of the system2104 as described, the R2 data storage system 2104 may return anacknowledgement to the R1 data storage system 2102 that it has receivedthe write data. Responsive to receiving this acknowledgement from the R2data storage system 2104, the R1 data storage system 2102 may return anacknowledgement to the host 2110 a that the write has been received andcompleted. Thus, generally, R1 LSU 2124 and R2 LSU 2126 may be logicaldevices, such as LUNs, configured as mirrors of one another. R1 and R2LSUs may be, for example, fully provisioned LUNs, such as thick (i.e.,not thin or virtually provisioned) LUNs, or may be LUNs that are thin orvirtually provisioned logical devices.

When operating in asynchronous mode when processing a received write I/Ooperation from a host as noted above, the primary or R1 data storagesystem 2102 may store the write data in its cache at a cache locationand mark the cache location as including write pending (WP) data asmentioned elsewhere herein. The write data may be propagated across anestablished RDF link (more generally referred to as a the remotereplication link or link) such as over 2108 b, 2122, and 2108 c, to thesecondary or R2 data storage system 2104 where the write data may bestored in the cache of the system 2104 at a cache location that ismarked as WP. Once the write data is stored in the cache of the system2104 as described, the R2 data storage system 2104 may return anacknowledgement to the R1 data storage system 2102 that it has receivedthe write data. With asynchronous mode, once the write data is stored inthe cache of the local or R1 system 2102 and marked as WP, anacknowledgement regarding completion of the host write may be sent tothe host 2110 a by the system 2102. Thus, in asynchronous mode thesystem 2102 is not required to wait to receive the acknowledgement fromthe R2 data storage system 2104 prior to sending the acknowledgement tothe host regarding completion of the write operation.

With reference to FIG. 6B, shown is a further simplified illustration ofcomponents that may be used in an embodiment in accordance withtechniques herein. The example 2400 is simplified illustration ofcomponents as described in connection with FIG. 2. Element 2402generally represents the replication link used in connection withsending write data from the primary R1 data storage system 2102 to thesecondary R2 data storage system 2104. Link 2402, more generally, mayalso be used in connection with other information and communicationsexchanged between 2101 and 2104 for replication. As mentioned above,when operating in synchronous replication mode, host 2110 a issues awrite, or more generally, all I/Os including reads and writes, over apath to only the primary R1 data storage system 2102. The host 2110 adoes not issue I/Os directly to the R2 data storage system 2104. Theconfiguration of FIG. 6B may also be referred to herein as anactive-passive configuration such as may be used with synchronousreplication and other supported replication modes where the host 2110 ahas an active connection or path 2108 a over which all I/Os are issuedto only the R1 data storage system. The host 2110 a may have a passiveconnection or path 2404 to the R2 data storage system 2104. In theconfiguration of 2400, the R1 LSU 2124 and R2 LSU 2126 may be configuredand identified as the same LSU, such as LSU A, to the host 2110 a. Thus,the host 2110 a may view 2108 a and 2404 as two paths to the same LSU Awhere path 2108 a is active (over which I/Os may be issued to LSU A) andwhere path 2404 is passive (over which no I/Os to the LSU A may beissued). Should the connection 2108 a and/or the R1 data storage system2102 experience a failure or disaster whereby access to R1 2124configured as LSU A is unavailable, processing may be performed on thehost 2110 a to modify the state of path 2404 to active and commenceissuing I/Os to the R2 LSU configured as LSU A. In this manner, the R2LSU 2126 configured as LSU A may be used as a backup accessible to thehost 2110 a for servicing I/Os upon failure of the R1 LSU 2124configured as LSU A.

It should be noted although only a single RDF link 2402 is illustrated,more generally any number of RDF links may be used in connection withreplicating data from systems 2102 to system 2104 in connection withtechniques herein.

Referring to FIG. 6C, shown is another example configuration ofcomponents that may be used in an embodiment in accordance withtechniques herein. The example 2500 illustrates an active-activeconfiguration as may be used in connection with synchronous replicationin at least one embodiment in accordance with techniques herein. In anactive-active configuration with synchronous replication, the host 2110a may have a first active path 2108 a to the R1 data storage system andR1 LSU 2124 configured as LSU A. Additionally, the host 2110 a may havea second active path 2504 to the R2 data storage system and R2 LSU 2126configured as LSU A. From the view of the host 2110 a, paths 2108 a and2504 appear as 2 paths to the same LSU A as described in connection withFIG. 6B with the difference that the host in the example 2500configuration may issue I/Os, both reads and/or writes, over both ofpaths 2108 a and 2504. The host 2110 a may send a first write over path2108 a which is received by the R1 system 2102 and written to cache ofthe R1 system 2102 where, at a later point in time, the first write isde-staged from the cache of the R1 system 2102 to physical storageprovisioned for the R1 LSU 2124 configured as LSU A. The R1 system 2102also sends the first write to the R2 system 2104 over link 2402 wherethe first write is written to cache of the R2 system 2104, where, at alater point in time, the first write is de-staged from the cache of theR2 system 2104 to physical storage provisioned for the R2 LSU 2126configured as LSU A. Once the first write is written to the cache of theR2 system 2104, the R2 system 2104 sends an acknowledgement over link2402 to the R1 system 2102 that it has completed the first write. The R1system 2102 receives the acknowledgement from the R2 system 2104 andthen returns an acknowledgement to host 2110 a over path 2108 a that thefirst write has completed.

The host 2110 a may also send a second write over path 2504 which isreceived by the R2 system 2104 and written to cache of the R2 system2104 where, at a later point in time, the second write is de-staged fromthe cache of the R2 system 2104 to physical storage provisioned for theR2 LSU 2126 configured as LSU A. The R2 system 2104 also sends thesecond write to the R1 system 2102 over a second link 2502 where thesecond write is written to cache of the R1 system 2102, and where, at alater point in time, the second write is de-staged from the cache of theR1 system 2102 to physical storage provisioned for the R1 LSU 2124configured as LSU A. Once the second write is written to the cache ofthe R1 system 2102, the R1 system 2102 sends an acknowledgement overlink 2502 to the R2 system 2104 that it has completed the second write.Once the R2 system 2104 receives the acknowledgement from the R1 system(regarding completion of the second write), the R2 system 2104 thenreturns an acknowledgement to host 2110 a over path 2504 that the secondwrite has completed.

Thus, in the example 2500, the illustrated active-active configurationincludes a first RDF R1-R2 LSU pairing configured for synchronousreplication (from 2102 to 2104) where the R1 LSU is 2124 and the R2 LSUis 2126 whereby writes to LSU A sent over 2108 a to system 2102 arestored on the R1 LSU 2124 and also transmitted to system 2104 over 2402.The write sent over 2402 to system 2104 is stored on the R2 LSU 2126.Such replication is performed synchronously in that the acknowledgementto the host write sent over 2108 a is not acknowledged as successfullycompleted unless and until the write data has been stored in caches ofsystems 2102 and 2104.

In a similar manner, the illustrated active-active configuration of theexample 2500 includes a second RDF R1-R2 LSU pairing configured forsynchronous replication (from 2104 to 2102) where the R1 LSU is 2126 andthe R2 LSU is 2124 whereby writes to LSU A sent over 2504 to system 2104are stored on the LSU 2126 (now acting as the R1 LSU of the second RDFLSU pairing) and also transmitted to system 2102 over connection 2502.The write sent over 2502 is stored on the R2 LSU 2124. Such replicationis performed synchronously in that the acknowledgement to the host writesent over 2504 is not acknowledged as successfully completed unless anduntil the write data has been stored in caches of systems 2102 and 2104.

Effectively, using the second RDF LSU pairing in the active-activeconfiguration with synchronous replication as in FIG. 6C has the R2system 2104 act as another primary data storage system which facilitatespropagation of writes received at the data storage system 2104 to thedata storage system 2102. It should be noted that although FIG. 6Cillustrates for simplicity a single host accessing both the R1 LSU 2124and R2 LSU 2126, any number of hosts may access one or both of the R1LSU 2124 and the R2 LSU 2126.

Although only a single RDF link 2402 is illustrated in connection withreplicating data from systems 2102 to system 2104 in connection withtechniques herein, more generally any number of RDF links may be used.Although only a single RDF link 2502 is illustrated in connection withreplicating data from systems 2104 to system 2102, more generally anynumber of RDF links may be used. Furthermore, although 2 RDF links 2402and 2502 are illustrated, in at least one embodiment, a single RDF linkmay be used in connection with sending data from system 2102 to 2104,and also from 2104 to 2102.

In at least one embodiment in accordance with techniques herein, the FCprotocol may be used in connection with communications (e.g., over theSAN including the RDF links) between the data storage system 2102 and2104.

Asynchronous remote replication (ARR) may include a plurality ofreplication cycles for an LSU (R1) in a source (e.g.,) storage system(A1), each cycle corresponding to a period of time and specifying any R1tracks for which data was updated (e.g., by a write operation) duringthe period of time represented by the cycle. Each cycle may transitionthrough four phases—two phases on A1 and two phases on a target (e.g.,secondary) storage system A2. The two phases on A1 may include a capturephase during which the data updates for R1 are captured or collected,after which the cycle transitions into a transfer phase during which theupdated data for R1 is transmitted from A1 to A2 to be updated for atarget LSU (R2) on A2. On A2, the cycle begins in a receive phase duringwhich the data updates transmitted as part of the transfer phase on A1are received on A2, and transitions to an apply phase during which thedata updates are applied to R2. Replication cycles for remotereplication are described in greater detail in U.S. Pat. No. 9,880,946,“Data Transfer Techniques with Data Replication,” to Benjamin Yoder etal., issued Jan. 30, 2018, the entire content of which is herebyincorporated by reference in its entirety.

In addition to employing remote replication techniques, embodiments ofthe invention may employ snapshot techniques, for example, as will nowbe described.

Referring to FIG. 7A, a replication data pointers (RDP) table 100includes a first linked list 102 of a plurality of logical storageelement (LSE) numbers 104 a-104 c, according to embodiments of theinvention. A logical storage element or LSE may be any logically definedportion of an LSU, including any of: a logical data unit (as definedelsewhere herein), a track (as defined elsewhere herein), an extent orother type of portion. The RDP table 100 may be used to maintain datathat is moved in connection with providing targetless snapshots, asdescribed herein. Each of the LSE numbers 104 a-104 c may correspond toan LSE of an LSU. The LSU may be, for example, a conventional logicaldevice with all of the LSEs having corresponding physical data storageallocated thereto or may be a thin device, as described in more detailelsewhere herein.

Each of the LSE numbers 104 a-104 c may correspond to one or more tableentries that are maintained using an appropriate data structure, such asa linked list. The LSE number 104 a may correspond to a plurality oftable entries 106 a-108 a, the LSE number 104 b may correspond to aplurality of table entries 106 b-108 b, and the LSE number 104 c maycorrespond to a plurality of table entries 106 c-108 c. Note that,although the table 100 is illustrated with three LSE numbers 104 a-104 ceach having three table entries, the table 100 may contain any number ofLSE numbers each having any number of table entries. In some cases,which will become apparent from the additional discussion herein, it ispossible for there to be no LSE number or corresponding table entriesassociated with a particular LSE of an LSU. Each of the table entries106 a-108 c may include a sequence number and a pointer to storage,which are explained in more detail elsewhere herein.

Referring to FIG. 7B, a replication data pointers (RDP) tree 110 mayinclude a plurality of table entries 112 a-112 f that each correspond toa particular LSE, according to embodiments of the invention. Each of thetable entries 112 a-112 f may include a sequence number and a pointer tostorage. The RDP tree 110 may correspond to one of the linked listspointed to by one of the data pointers 104 a-104 c of the RDP table 100,discussed above. The sequence number and the pointer to storage may besimilar to the sequence number and pointer to storage used in connectionwith the RDP table 100, and are described in more detail elsewhereherein. In an embodiment herein, the RDP tree 110 is a balanced binarytree ordered according to sequence number.

Referring to FIG. 8, a data pool 115 may include storage for data thatis moved in connection with maintaining targetless snapshots, accordingto embodiments of the invention. Data stored in the data pool 115 may bepointed to by the pointers provided with the table entries 106 a-108 cor the table entries 112 a-112 f In some embodiments, the data pool 115is provided in a single logical and/or physical location. In otherembodiments, the data pool 115 may be distributed and/or may use morethan one physical and/or logical data storage element. Providing data tothe data pool 115 is discussed in more detail elsewhere herein.

Referring to FIG. 9, a snapshot table 120 may include a plurality ofentries corresponding to particular snapshots, according to embodimentsof the invention. Each of the entries may include a snapshot ID and asequence number. The snapshot ID may be used to identify a particularsnapshot and could be text (e.g., “March 12, 2014, 8:00 am snapshot”) orcould be a token that is used by other software (not shown herein) toidentify each of the snapshots. The sequence number provided with eachof the snapshots may be used in connection with providing targetlesssnapshots and is described in more detail elsewhere herein.

Referring to FIG. 10, a sequence number table 130 is shown as having aplurality of entries, according to embodiments of the invention. Each ofthe entries of the table 130 may contain a sequence number, as describedin more detail elsewhere herein. The table 130 may contain a singleentry for each LSE number (or other appropriate data increment) of theLSU (e.g., logical device or thin device) for which targetless snapshotsare being provided. Thus, for example, if there are one hundred LSEs inan LSU, there may be one hundred entries for sequence numbers in thetable 130. Use of the sequence number table 130 and of sequence numbersis described in more detail elsewhere herein.

FIG. 11 is a flowchart illustrating an example of a method 1100 ofperforming operations in connection with performing targetless snapshotsfor a LSU, according to embodiments of the invention. In a step 1102, aglobal sequence number (associated with the LSU for which targetlesssnapshots are being provided) and the tables 100, 120, 130 that are usedwith targetless snapshots may be initialized. Note that the RDP tree 110may be used in addition to or instead of the RDP table 100. In anembodiment herein, snapshot sequence numbers start at zero and areincremented by one for each snapshot, but of course in other instancesit is possible to start at any number and increment or decrement by anyamount. At the step 1102, the RDP table 100 (and/or the RDP tree 110)may be initialized to be empty (contain no entries), the snapshot table120 may be initialized to be empty, the sequence number table 130 may beinitialized so that each entry contains zero (the initial sequencenumber), and the global sequence number may be initialized to zero (theinitial sequence number).

Following the step 1102 may be a step 1104 where the system waits for asnapshot to occur. A snapshot may be user initiated or may be automatedto occur at specific times (e.g., every hour). Once a snapshot occurs,control may transfer from the step 1104 to a step 1106 where an entrycorresponding to the snapshot may be created in the snapshot table 120.At the step 1106, the ID value may be provided to the new entry in thesnapshot table 120 and the corresponding sequence number may be set toone greater than the current global sequence number. The ID value mayinclude a user specified name that is to be associated with the sequencenumber provided to the entry. Following the step 1106 may be a step 1108where the global sequence number is incremented. Following the step1108, control may transfer back to the step 1104 to wait for the nextsnapshot to occur.

FIG. 12 is a flowchart illustrating an example of a method 1220performed in connection with a write operation to a LSU for whichsnapshots are being provided, according to embodiments of the invention.In a test step 1222, it may be determined if the global sequence numberequals the sequence number associated with the LSE to which the write isbeing provided, which may be provided by the sequence number table 130.If so, then control may transfer from the test step 1222 to a step 1224where the write operation may be performed in a usual fashion. Nospecial processing may be performed in this case because the globalsequence number being equal to the sequence number where the data isbeing written means that any snapshot data associated with thatparticular data section has already been protected (copied to the datapool 115, as described in more detail elsewhere herein). Following thestep 1224, processing may be complete.

If it is determined in the step 1222 that the global sequence numberdoes not equal the sequence number associated with the LSE to which thewrite is being performed (the global sequence number is greater), thencontrol may transfer from the step 1222 to a step 1226 where an entry inthe RDP table 100 may be created by placing the new entry in a linkedlist using the LSE number where the write is being performed on the LSUand using the sequence number for the source LSE (from the sequencenumber table 130). If the RDP tree 110 is used, then in the step 1226 anew entry may be created for the RDP tree 110. Following the step 1226may be a step 1228 where data that is being overwritten is copied fromthe LSU to the data pool 115. Note that the step 1228 may be omitted ininstances where the LSU is a thin device and the particular LSE is empty(e.g., the pointer for the LSE points to null). Note also that, in somecases data on the LSU may be cached, in which case the copy may be fromthe cache memory.

Following the step 1228 is a step 1232 where the pointer in the tableentry created at the step 1226, described above, may be set to point tothe data in the data pool 115 that was copied at the step 1228,described above, or to null in the case of a thin logical device with nodata in the LSE. Following the step 1232 is a step 1234 where thesequence number for the entry in the sequence number table 130 may beset to the global sequence number, indicating that the correspondingdata written to the LSU corresponds to the current global sequencenumber. Following the step 1234 may be the step 1224, discussed above,where the write operation to write the new data to the device may beperformed. Following the step 1224, processing may be complete.

FIG. 13 is a flowchart illustrating an example of a method 1350 ofprocessing performed in connection with reading different versions fromdifferent snapshots of data on the LSU, according to embodiments of theinvention. In a step 1352, it may be determined if a sequence numberassociated with a desired version (VER in flow diagram 1350) is greaterthan or equal to a version number from the sequence number table (SNT inthe flow diagram 1350). For example, if it was desired to read a versionof data associated with a particular snapshot (e.g., “8:00 am on Mar.12, 2014”) having a sequence number X, then the test at the step 1352may compare X with an entry in the sequence number table for the LSE ofinterest containing data being read, which may be provided in thesequence number table 130. If it is determined in the step 1352 that thesequence number of the desired version is greater than or equal to aversion number from the sequence number table corresponding to the databeing read, then data on the LSU was written prior to the time of thesnapshot, and control may transfer from the step 1352 to the step 1354where the data is read from the LSU. Note that this also may occur whenit is desired to read current data from the LSU since data on thelogical volume should always be the latest version. Following the step1354, processing may be complete.

If it is determined at the step 1352 that the sequence number of thedesired version is not greater than or equal to a version number fromthe sequence number table corresponding to the data being read, thendata on the LSU was written after the time of the snapshot and thedesired data is in the data pool 115, and control may transfer from thestep 1352 to a step 1356 where an iteration pointer may be set to pointto a first item in a list of items in the RDP table 100. The iterationpointer may be used to traverse a list of pointers for a LSEcorresponding to the data being read. For the explanation herein, it maybe assumed that the list of pointers is arranged with the most recentlyadded table entry (having the highest sequence number) being first inthe list, followed by the next most recently added table entry (havingthe second highest sequence number), etc. Generally, the iterationpointer may iterate through table entries for a specific LSE fromhighest sequence number to lowest sequence number. Note that, ininstances where the RDP tree 110 is used, the iteration pointer may beset to point to the top of the RDP tree 110 and is used to traverse theRDP tree 110.

Following the step 1356 may be a test step 1358 where it may bedetermined if a sequence number associated with the desired version isgreater than or equal to a version number associated with the table ortree entry indicated by the iteration pointer, similar to the test atthe step 1352, discussed above. If so, then control may transfer fromthe test step 1358 to a step 1362 where data may be read from the datapool 115 according to the data pointer of the RDP table or RDP treeentry indicated by the iteration pointer. Following the step 1362,processing may be complete. Otherwise, if it is determined at the step1358 that the sequence number associated with the desired version is notgreater than or equal to the version number associated with the table ortree entry indicated by the iteration pointer, then control may transferfrom the step 1358 to a step 1364 where the iteration pointer is set topoint to a next table or tree entry. Note that the final item of thetable or tree entries may have a sequence number of zero so that,eventually, the test at the step 1358 will cause the step 1362 to beexecuted.

In some instances, it is possible to maintain written data in memory(e.g., in a cache database in the global memory 26). Version informationmay be maintained with the written data in memory to facilitateeventually moving the data to the LSU while providing targetlesssnapshots as described herein. The data may be moved using a backgroundprocess. Memory may be employed in this manner as described in theJaganathan patent.

In some embodiments of the invention, a first LSU (R1) on a firststorage system (A1), which may be considered a primary storage system,may be remotely replicated to a second LSU (R2), which may be referredto herein as a replica LSU, on a second storage system (A1), which maybe considered a secondary storage system.

FIG. 14 is a flow diagram illustrating an example of a method 1400 oflogging write operations for remote replication of a snapshot, accordingto embodiments of the invention. Other embodiments of a method oflogging write operations for remote replication of a snapshot, forexample, variations of the method 1400, are possible and are intended tofall within the scope of the invention. One or more components of A1 andA2, for example, one or more of the directors 37 a-n or directors 216a-n described in more detail in relation to FIGS. 1 and 2 may beconfigured to collectively implement the method 1400.

In a step 1402, A1 may suspend initiating processing of new writerequests from any host system, for example, starting at the commencementof a consistency window at a time, T1. During the consistency window, A1may record any write operations of write requests of R1 that wereoutstanding at the time T1 (and other LSUs) in a step 1404. In a step1406, outstanding write requests of R1 may be recorded, for example, ina data structure 1500 illustrated in FIG. 15.

FIG. 15 is a block diagram illustrating an example of a data structure1500 for logging write operations (e.g., of outstanding write requests)for remote replication of a snapshot, according to embodiments of theinvention. Other embodiments of a data structure for logging writeoperations for remote replication of a snapshot, for example, variationsof the data structure 1500, are possible and are intended to fall withinthe scope of the invention. The data structure 1500 may be referred toherein as an outstanding write log, and may be implemented in memory 26,e.g., global memory 25 b.

The outstanding write log 1500 may include a plurality of entries 1502,where each of the plurality of entries 1502 may represent an outstandingwrite request. Each entry may include: an LSU ID of the LSUcorresponding to the represented outstanding write request in an LSUcolumn 1504; a snapshot ID of the snapshot corresponding to therepresented outstanding write request in a snapshot (SS) column 1506; anLSE ID of the LSE specified by the represented outstanding write requestin an LSE column 1512; a data value specified by the representedoutstanding write request in a data column 1514; and other informationcorresponding to the represented outstanding write request in one ormore other information columns 1516.

In some embodiments, the entries 1502 may be indexed by LSU ID, snapshotID and/or LSE ID. One or more other data structures, including otherindexes, may be created from the outstanding write log 1500. Other typesof data structures may be used to log or otherwise store outstandingwrite information, for example, object-oriented data structures, linkedlists or trees.

The outstanding write log may include outstanding write operations formultiple LSUs on a storage system (e.g., A1), and may includeoutstanding write operations for several snapshots of a same LSU, forexample, several snapshots taken during a same consistency window for anLSU.

Returning to FIG. 14, a first snapshot, SS1 ₁, may be taken on A1 in astep 1406, and A1 may initiate taking a second snapshot, SS1 ₂, which isintended to be a replica of SS1 ₁, on A2. A1 may initiate SS1 ₂ bysending an instruction to A2, where the instruction may includeinformation about SS1 ₁, including the SS1 ₁ ID. In response toreceiving the instruction, A2 may take SS1 ₂, and acknowledge to A1 theactivation of SS1 ₂ ID, which may include information about SS1 ₂,including the SS1 ₂ ID. At least initially, SS1 ₂ may not be an exactreplica of SS1 ₁ given that SS1 ₂ may have been taken before one or morewrite operations of R1 have been replicated to R2.

In response to receiving the acknowledgement from A2, A1 may close theconsistency window and resume initiating processing of new writerequests received from any host system for R1. In some embodiments, thesteps 1402-1412 may be considered part of Phase 1 of remotelyreplicating snapshots.

In a step 1414, the outstanding write requests of SS1 ₁ may be appliedto SS1 ₂, for example, as described in more detail elsewhere herein. Theoutstanding write requests may be applied concurrently to the ongoingreplication of write requests from R1 to R2, for example as part of aseparate (e.g., background) process. The step 1414 may be consideredpart of Phase 2 of remotely replicating snapshots.

Applying the outstanding write requests for SS1 ₁ in Phase 2 may includeprocessing on A1 and A2. FIG. 16 is a flow diagram illustrating anexample of a method 1600 performed by a first storage system, A1, aspart of remotely replicating snapshots from R1 to a second storagesystem, A2, according to embodiments of the invention. Other embodimentsof a method performed by A1 as part of remotely replicating snapshotsfrom R1 to R2, for example, variations of the method 1600, are possibleand are intended to fall within the scope of the invention. The method1600 may be performed by A1 as part of the Phase 2 processing. One ormore components of A1, for example, one or more of the directors 37 a-nor directors 216 a-n described in more detail in relation to FIGS. 1 and2 may be configured to collectively implement the method 1600.

In a step 1602, it may be determined whether there is a next outstandingwrite request to process for an LSE of R1 for SS1 ₁, for example, byaccessing SS1 ₁ outstanding write entries in the outstanding write log1500. If there is not a next outstanding write entry, the method 1600may end. Otherwise, in a step 1604, it may be determined whether R1 LSEmetadata points to the current data of the R1 LSE; i.e., points to asame storage location as the current R1 LSE metadata. The snapshotmetadata of the R1 LSE may include the sequence number table 130, RDPtable 100 and/or RDP tree 110 described elsewhere herein or inJaganathan or the snapshot look-up tables (SLTs), current look-up tables(CLTs) and snapshot pointer structures (SPSs) described in Tobin. Themetadata for the current value of the R1 LSE may include the master LSUtable 62, the LSU table 72, the thin device table 72′ and the LSU tracktable 82 described in more detail elsewhere herein.

If the R1 LSE metadata points to the current data of the R1 LSE, then,in a step 1606, it may be determined whether the remote replication ofany write operations for the R1 LSE are still pending. If so, Phase 2processing of any outstanding write requests of R1, or perhaps only theLSE in particular, may be suspended until acknowledgements are receivedfrom A2 that the outstanding write requests for the R1 LSE have beencompleted (e.g., cached or stored on a physical storage device) on A2.

In some embodiments, rather than waiting for the replication-pendingwrites of the R1 LSE to complete, it may be may recorded (e.g., in adata structure) that the R1 LSE has replication-pending writes, and thePhase 2 processing of other R1 LSEs on A1 may continue. A list of R1LSEs having replication-pending writes may be maintained on A1. AfterPhase 2 processing for the last R1 LSE not having replication-pendingwrites is completed, the Phase 2 processing may return to the R1 LSEshaving replication-pending writes, for example, by accessing the list ofreplication-pending LSEs. Returning to the R1 LSEs havingreplication-pending writes may be done in consideration that thereplication-pending write operations of the LSEs may have beenreplicated to R2 (as part of typical replication processing) during thetime that has elapsed since last checked, so that the previously notedreplication-pending writes have now been acknowledged by A2 as complete.For any R1 LSE that still has a replication-pending write, the processmay move on to the next R1 LSE of the list and attempt further Phase IIprocessing of the R1 LSE, and repeat the foregoing process until thereare no longer any replication-pending LSEs.

If it is determined in the step 1604 that R1 LSE metadata does not pointto the current data of the R1 LSE, or after it is determined in the step1606 that there are not any replication-pending writes for the R1 LSE,A1 may send outstanding write information about the outstanding writeoperation in an OWI to A2 in a step 1608. The OWI may specify any of: anID of SS1 ₁; an ID of the R1 LSE; an ID of R2; and an indication ofwhether the value of the R1 LSE metadata for SS1 ₁ is the current valueof R1 LSE, i.e., points to the storage location of the current value ofR1 LSE, or is a value of snapshot data for R1 LSE for SS1 ₁, i.e.,points to a location of snapshot data specific to R1 LSE for SS1 ₁.

FIG. 17 is a flow diagram illustrating an example of a method 1700performed by a second storage system, R2, as part of remotelyreplicating snapshots from a first storage system, A1, to the secondstorage system, A2, according to embodiments of the invention. Otherembodiments of a method performed by R2 as part of remotely replicatingsnapshots from R1 to R2, for example, variations of the method 1700, arepossible and are intended to fall within the scope of the invention. Themethod 1700 may be performed by A2 as part of the Phase 2 processing.One or more components of A2, for example, one or more of the directors37 a-n or directors 216 a-n described in more detail in relation toFIGS. 1 and 2 may be configured to collectively implement the method1700.

In a step 1702, A2 may receive the OWI for the R1 LSE from A1, which mayinclude, among other information, the R1 LSE state indicating whetherthe LSE SS1 ₁ metadata points to the current data of the R1 LSE orpoints to snapshot data of the R1 LSE.

In a step 1704, A2 may determine, for example, from the OWI, whether theSS1 ₁ LSE value is the current LSE value; i.e., whether the LSE SS1 ₁metadata points to the current data of the R1 LSE. Alternatively, thestep 1704 may determine whether the SS1 ₁ LSE value is a value ofsnapshot data for R1 LSE for SS1 ₁; i.e., whether the R1 LSE metadatafor SS1 ₁ points to a location of snapshot data specific to R1 LSE forSS1 ₁.

If it is determined that the SS1 ₁ LSE value is the current LSE value(or alternatively that the SS1 ₁ LSE value is not a snapshot data valuefor R1 LSE for SS1 ₁), then it may be determined in a step 1706 whetherthe LSE SS1 ₂ metadata points to the current data of the R2 LSE.Alternatively, the step 1706 may determine whether the SS1 ₂ LSE valueis a value of snapshot data for R2 LSE for SS1 ₁; i.e., whether the R2LSE metadata for SS1 ₁ points to a location of snapshot data specific toR2 LSE for SS1 ₂. The snapshot metadata of the R2 LSE may include thesequence number table 130, RDP table 100 and/or RDP tree 110 describedelsewhere herein or in Jaganathan or the snapshot look-up tables (SLTs),current look-up tables (CLTs) and snapshot pointer structures (SPSs)described in Tobin. The metadata for the current value of the R2 LSE mayinclude the master LSU table 62, the LSU table 72, the thin device table72′ and the LSU track table 82 described in more detail elsewhereherein.

If it is determined in the step 1706 that the LSE SS1 ₂ metadata pointsto the current data of the R2 LSE (or alternatively that the SS1 ₂ LSEvalue is not a value of snapshot data for R2 LSE for SS1 ₁), then SS1 ₂LSE metadata points to the same current data as the SS1 ₁ LSE metadata.That is, as the SS1 ₁ LSE metadata points to the current data of the R1LSE value, and the process described in relation to the step 1606ensured that, if the SS1 ₁ LSE metadata points to the current data ofthe R1 LSE value, any replication-pending writes were completed on R2before the OCI was sent, this means that there are not any R1 LSE writeoperations that have not reached R2 LSE yet. Thus, because both SS1 ₁LSE metadata and SS1 ₂ LSE metadata point to their respective currentLSE values, these values are the same values. Accordingly, if it isdetermined in the step 1706 that the LSE SS1 ₂ metadata points to thecurrent data of the R2 LSE, the method may end.

If it is determined in the step 1706 that the LSE SS1 ₂ metadata doesnot point to the current data of the R2 LSE (or alternatively that theSS1 ₂ LSE value is a value of snapshot data for R2 LSE for SS1 ₁), thismeans that the outstanding write operation specified in the OWI receivedin the step 1702 was performed on R1 before SS1 ₁ was taken but wasperformed on R2 after SS1 ₂ was taken. Accordingly, if it is determinedin the step 1706 that the LSE SS1 ₂ metadata does not point to thecurrent data of the R2 LSE, A2 may update the SS1 ₂ metadata to point tothe current value of the R2 LSE in a step 1712.

It should be noted that, if it is determined in the step 1706 that theLSE SS1 ₂ metadata does not point to the current data of the R2 LSE,then the LSE SS1 ₂ metadata points to snapshot data for R2 LSE for SS1₁, and it should not be possible for this snapshot data to be the valueof the outstanding write data of the OWI received in the step 1702.However, it may be desirable to compare this snapshot data to the valueof the outstanding write data as a kind of sanity check. Accordingly, ina some embodiments, if it is determined in the step 1706 that the LSESS1 ₂ metadata does not point to the current data of the R2 LSE (oralternatively that the SS1 ₂ LSE value is a value of snapshot data forR2 LSE for SS1 ₁), then it may be determined in a step 1708 whether thesnapshot data for R2 LSE for SS1 ₁ has the same value as the outstandingwrite data of the OWI, and if so, an error may be reported. Otherwise,the method may proceed to the step 1712.

Returning to the step 1704, if it is determined that the SS1 ₁ LSE valueis not the current LSE value (or alternatively that the SS1 ₁ LSE valueis a value of snapshot data for R1 LSE for SS1 ₁), this means that awrite operation was performed on the LSE after SS1 ₁ was taken. Thisperformed on then in a step 1714, and in a step 1706 it may bedetermined whether the LSE SS1 ₂ metadata points to the current data ofthe R2 LSE (or alternatively whether the SS1 ₂ LSE value is a value ofsnapshot data for R2 LSE for SS1 ₁).

As it is known from the result of step 1714 that a write operation wasperformed on the R1 LSE after SS1 ₁ was taken, if it is determined inthe step 1714 that the LSE SS1 ₂ metadata points to the current data ofthe R2 LSE (or alternatively that the SS1 ₂ LSE value is not a value ofsnapshot data for R2 LSE for SS1 ₁), this means that the write operationperformed on the R1 LSE after SS1 ₁ was taken has not been performed onR2 yet. However, the outstanding write operation specified by the OCImay not be the write operation performed on the R1 LSE after SS1 ₁ wastaken, but may be an earlier write operation performed on the R1 LSEbefore SS1 ₁ was taken, which also has not yet been performed on R2 yet.Accordingly, in an embodiment, if it is determined in the step 1714 thatthe LSE SS1 ₂ metadata points to the current data of the R2 LSE, thestep 1714 may be repeated (i.e., the method 1700 may wait) until it isdetermined in the step 1714 that that the LSE SS1 ₂ metadata does notpoint to the current data of the R2 LSE (or alternatively that the SS1 ₂LSE value is a value of snapshot data for R2 LSE for SS1 ₁), in whichcase it may be determined in a step 1716 whether the snapshot data forR2 LSE for SS1 ₁ has the same value as the outstanding write data of theOWI.

In some embodiments, rather than waiting until it is determined in thestep 1714 that that the R2 LSE SS1 ₂ metadata does not point to thecurrent data of the R2 LSE, it may be may recorded (e.g., in a datastructure) that the R2 LSE SS1 ₂ has replication-pending writes, and themethod 1700 may end for the R2 LSE SS1 ₂ (and performed again for anynew OWIs received). A list of R2 LSEs having replication-pending writesmay be maintained on A2. After a first pass of the method 1700 isperformed for any OCIs for R2, the method 1700 may be performed againfor any R2 LSEs determined to have replication-pending writes on anearlier (e.g., first) pass, for example, by accessing the list ofreplication-pending LSEs. Returning to the R2 LSEs havingreplication-pending writes may be done in consideration that thereplication-pending write operations of the LSEs may have beenreplicated to R2 (as part of typical replication processing) during thetime that has elapsed since last checked, so that the previously notedreplication-pending writes have now been performed on R2. For any R2 LSEthat still has a replication-pending write on a given pass through themethod 1700, the method 1700 may be performed on to the next R2 LSE ofthe list, and the foregoing process may be repeated until there are nolonger any replication-pending LSEs for R2.

If it is determined in the step 1716 that the snapshot data has the samevalue as the outstanding write data, this means that the LSE SS1 ₂metadata and the LSE SS1 ₁ metadata point to a same snapshot value,meaning that the outstanding write operation of the OWI was receivedafter SS1 ₁ was taken on A1 and after SS1 ₂ was taken on A2, but hasalready been applied to both R1 and R2. Accordingly, if it is determinedin the step 1716 that the snapshot data for R2 LSE for SS1 ₁ has thesame value as the outstanding write data of the OWI, the method 1700 mayend.

If it is determined in the step 1716 that the snapshot data for R2 LSEfor SS1 ₁ does not have the same value as the outstanding write data ofthe OWI, this means that: a) an earlier write operation was performed onR1 LSE before SS 1 ₁ was taken, but was performed on R2 LSE after SS1 ₂was taken; and b) the outstanding write operation of the OWI wasperformed on R1 LSE after SS1 ₁ was taken, but has not yet beenperformed on R2 LSE. As a result, the LSE SS1 ₂ metadata points to wrongsnapshot data, and should be fixed.

Accordingly, if it is determined in the step 1716 that the snapshot datafor R2 LSE for SS1 ₁ does not have the same value as the outstandingwrite data of the OWI, A2 may allocate a storage location (e.g., from apool of storage reserved for snapshot data) for the outstanding writedata of the OWI, and the outstanding write data may be stored at theallocated location in a step 1720. In a step 1722, the LSE SS1 ₂metadata may be modified to point to the allocated location, forexample, as described herein, in Jaganathan and/or in Tobin.

Various embodiments of the invention may be combined with each other inappropriate combinations. Additionally, in some instances, the order ofsteps in the flowcharts, flow diagrams and/or described flow processingmay be modified, where appropriate. It should be appreciated that any ofthe methods described herein, including methods 1100, 1200, 1300, 1400,1600 and 1700, or parts thereof, may be implemented using one or more ofthe systems and/or data structures described in relation to FIGS. 1-10and 15, or components thereof. Further, various aspects of the inventionmay be implemented using software, firmware, hardware, any suitablecombination thereof and/or other computer-implemented modules or deviceshaving the described features and performing the described functions.Logic that when executed performs methods described herein, stepsthereof or portions of such methods or steps, may be implemented assoftware, firmware, hardware, or any suitable combination thereof.

Software implementations of embodiments of the invention may includeexecutable code that is stored on one or more computer-readable mediaand executed by one or more processors. Each of the computer-readablemedia may be non-transitory and include a computer hard drive, ROM, RAM,flash memory, portable computer storage media such as a CD-ROM, aDVD-ROM, a flash drive, an SD card and/or other drive with, for example,a universal serial bus (USB) interface, and/or any other appropriatetangible or non-transitory computer-readable medium or computer memoryon which executable code may be stored and executed by a processor.Embodiments of the invention may be used in connection with anyappropriate OS.

As used herein, an element or operation recited in the singular andpreceded with the word “a” or “an” should be understood as not excludingplural elements or operations, unless such exclusion is explicitlyrecited. References to “one” embodiment or implementation of the presentdisclosure are not intended to be interpreted as excluding the existenceof additional embodiments that also incorporate the recited features.Furthermore, a description or recitation in the general form of “atleast one of [a], [b] or [c],” or equivalent thereof, should begenerally construed to include [a] alone, [b] alone, [c] alone, or anycombination of [a], [b] and [c]. In addition, use of a an ordinal term,e.g., “first,” “second” or the like, to qualify a term for an itemhaving multiple instances of the same name does not necessarily indicatea priority, precedence or temporal order between the instances unlessotherwise indicated, but rather such ordinal terms may be used merely todistinguish between the separate instances.

Other embodiments of the invention will be apparent to those skilled inthe art from a consideration of the specification or practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with the true scope and spiritof the invention being indicated by the following claims.

What is claimed is:
 1. For a system including a host system, a firststorage system, a second storage system, a first logical storage unitfor which data is replicated from the first storage system to a secondlogical storage unit of the second storage system, a method of remotelyreplicating a first snapshot of the first logical storage unit from thefirst storage system to the second storage system, the methodcomprising: at a first point in time, suspending initiating ofprocessing on the first storage system of new write requests for thefirst logical storage unit received from the host system after the firstpoint in time; recording at least a first outstanding write request forthe first logical storage unit on the first storage system that isoutstanding at the first point in time; activating the first snapshot onthe first storage system; initiating activation on the second storagesystem of a second snapshot of the second logical storage unit as aremote replica of the first snapshot; after the recording, theactivating and the initiating: resuming the initiating of processing ofnew write requests for the first logical storage unit on the firststorage system, and applying the at least first outstanding writerequest to the second snapshot.
 2. The method of claim 1, wherein theapplying is performed as part of a process executing independently of,and concurrently with, one or more processes that are remotelyreplicating write operations of the first logical storage unit to thesecond logical storage unit.
 3. The method of claim 1, wherein applyingthe at least first outstanding write request includes, for eachoutstanding write request of the at least first outstanding writerequest: determining whether the outstanding write request requiresmodification of snapshot data of the second snapshot; and if it isdetermined that the outstanding write request requires modification ofthe snapshot, modifying the snapshot data and snapshot metadata of thesecond snapshot on the second storage system based on the outstandingwrite request.
 4. The method of claim 3, wherein the first outstandingwrite request specifies a write operation for a first logical storageelement corresponding to a second logical storage element of the secondlogical storage unit, and wherein determining whether the firstoutstanding write request requires modification of snapshot data of thesecond snapshot includes: determining whether first snapshot metadata ofthe first snapshot references a first current value of the first logicalstorage element on the first storage system; determining whether secondsnapshot metadata of the second snapshot references a second currentvalue of the second logical storage element on the second storagesystem; and determining whether to modify the snapshot data and snapshotmetadata of the second snapshot based at least in part on whether thefirst snapshot metadata references the first current value and whetherthe second snapshot metadata references the second current value.
 5. Themethod of claim 4, wherein determining whether the first outstandingwrite request requires modification of snapshot data of the secondsnapshot includes: determining that the first snapshot metadatareferences the first current value of the first logical storage element;determining that the second snapshot metadata does not reference thesecond current value of the second logical storage element; andmodifying the snapshot metadata of the second snapshot to reference thesecond current value of the second logical storage element.
 6. Themethod of claim 4, wherein determining whether the first outstandingwrite request requires modification of snapshot data of the secondsnapshot includes: determining whether a first value of the secondlogical storage element specified by the second snapshot metadata is asame value as a second value of the second logical storage elementspecified by the write operation; and determining whether to modify thesnapshot data and snapshot metadata of the second snapshot based atleast in part on whether the first value is the same value as the secondvalue.
 7. The method of claim 4, wherein determining whether the firstoutstanding write request requires modification of snapshot data of thesecond snapshot includes: determining that the first snapshot metadatadoes not reference the first current value of the first logical storageelement; determining that the second snapshot metadata does notreference the second current value of the second logical storageelement; storing the second value of the second logical storage elementat a location on the second storage system; and modifying the snapshotmetadata of the second snapshot to reference the location.
 8. A systemcomprising: a first storage system; a second storage system; a thirdstorage system; a logical storage unit synchronously replicated betweenthe first storage system and the second system, wherein the logicalstorage unit is asynchronously replicated from the first storage systemto the third storage system and asynchronously replicated from thesecond storage system to the third storage system; and executable logicthat implements a method including: at a first point in time, suspendinginitiating of processing on the first storage system of new writerequests for the first logical storage unit received from the hostsystem after the first point in time, recording at least a firstoutstanding write request for the first logical storage unit on thefirst storage system that is outstanding at the first point in time,activating the first snapshot on the first storage system, initiatingactivation on the second storage system of a second snapshot of thesecond logical storage unit as a remote replica of the first snapshot;after the recording, the activating and the initiating, resuming theinitiating of processing of new write requests for the first logicalstorage unit on the first storage system, and applying the at leastfirst outstanding write request to the second snapshot.
 9. The system ofclaim 8, wherein the applying is performed as part of a processexecuting independently of, and concurrently with, one or more processesthat are remotely replicating write operations of the first logicalstorage unit to the second logical storage unit.
 10. The system of claim8, wherein applying the at least first outstanding write requestincludes, for each outstanding write request of the at least firstoutstanding write request: determining whether the outstanding writerequest requires modification of snapshot data of the second snapshot;and if it is determined that the outstanding write request requiresmodification of the snapshot, modifying the snapshot data and snapshotmetadata of the second snapshot on the second storage system based onthe outstanding write request.
 11. The system of claim 8, wherein thefirst outstanding write request specifies a write operation for a firstlogical storage element corresponding to a second logical storageelement of the second logical storage unit, and wherein determiningwhether the first outstanding write request requires modification ofsnapshot data of the second snapshot includes: determining whether firstsnapshot metadata of the first snapshot references a first current valueof the first logical storage element on the first storage system;determining whether second snapshot metadata of the second snapshotreferences a second current value of the second logical storage elementon the second storage system; and determining whether to modify thesnapshot data and snapshot metadata of the second snapshot based atleast in part on whether the first snapshot metadata references thefirst current value and whether the second snapshot metadata referencesthe second current value.
 12. The system of claim 11, whereindetermining whether the first outstanding write request requiresmodification of snapshot data of the second snapshot includes:determining that the first snapshot metadata references the firstcurrent value of the first logical storage element; determining that thesecond snapshot metadata does not reference the second current value ofthe second logical storage element; and modifying the snapshot metadataof the second snapshot to reference the second current value of thesecond logical storage element.
 13. The system of claim 11, whereindetermining whether the first outstanding write request requiresmodification of snapshot data of the second snapshot includes:determining whether a first value of the second logical storage elementspecified by the second snapshot metadata is a same value as a secondvalue of the second logical storage element specified by the writeoperation; and determining whether to modify the snapshot data andsnapshot metadata of the second snapshot based at least in part onwhether the first value is the same value as the second value.
 14. Thesystem of claim 11, wherein determining whether the first outstandingwrite request requires modification of snapshot data of the secondsnapshot includes: determining that the first snapshot metadata does notreference the first current value of the first logical storage element;determining that the second snapshot metadata does not reference thesecond current value of the second logical storage element; storing thesecond value of the second logical storage element at a location on thesecond storage system; and modifying the snapshot metadata of the secondsnapshot to reference the location.
 15. For a system including a hostsystem, a first storage system, a second storage system, a first logicalstorage unit for which data is replicated from the first storage systemto a second logical storage unit of the second storage system,computer-readable media having software thereon defining a method ofremotely replicating a first snapshot of the first logical storage unitfrom the first storage system to the second storage system, the softwarecomprising: executable code that controls suspending, at a first pointin time, initiating of processing on the first storage system of newwrite requests for the first logical storage unit received from the hostsystem after the first point in time; executable code that controlsrecording at least a first outstanding write request for the firstlogical storage unit on the first storage system that is outstanding atthe first point in time; executable code that controls activating thefirst snapshot on the first storage system; executable code thatcontrols initiating activation on the second storage system of a secondsnapshot of the second logical storage unit as a remote replica of thefirst snapshot; executable code that controls, after the recording, theactivating and the initiating: resuming the initiating of processing ofnew write requests for the first logical storage unit on the firststorage system, and applying the at least first outstanding writerequest to the second snapshot.
 16. The computer-readable media of claim15, wherein applying the at least first outstanding write requestincludes, for each outstanding write request of the at least firstoutstanding write request: determining whether the outstanding writerequest requires modification of snapshot data of the second snapshot;and if it is determined that the outstanding write request requiresmodification of the snapshot, modifying the snapshot data and snapshotmetadata of the second snapshot on the second storage system based onthe outstanding write request.
 17. The computer-readable media of claim16, wherein the first outstanding write request specifies a writeoperation for a first logical storage element corresponding to a secondlogical storage element of the second logical storage unit, and whereindetermining whether the first outstanding write request requiresmodification of snapshot data of the second snapshot includes:determining whether first snapshot metadata of the first snapshotreferences a first current value of the first logical storage element onthe first storage system; determining whether second snapshot metadataof the second snapshot references a second current value of the secondlogical storage element on the second storage system; and determiningwhether to modify the snapshot data and snapshot metadata of the secondsnapshot based at least in part on whether the first snapshot metadatareferences the first current value and whether the second snapshotmetadata references the second current value.
 18. The computer-readablemedia of claim 17, wherein determining whether the first outstandingwrite request requires modification of snapshot data of the secondsnapshot includes: determining that the first snapshot metadatareferences the first current value of the first logical storage element;determining that the second snapshot metadata does not reference thesecond current value of the second logical storage element; andmodifying the snapshot metadata of the second snapshot to reference thesecond current value of the second logical storage element.
 19. Thecomputer-readable media of claim 17, wherein determining whether thefirst outstanding write request requires modification of snapshot dataof the second snapshot includes: determining whether a first value ofthe second logical storage element specified by the second snapshotmetadata is a same value as a second value of the second logical storageelement specified by the write operation; and determining whether tomodify the snapshot data and snapshot metadata of the second snapshotbased at least in part on whether the first value is the same value asthe second value.
 20. The computer-readable media of claim 17, whereindetermining whether the first outstanding write request requiresmodification of snapshot data of the second snapshot includes:determining that the first snapshot metadata does not reference thefirst current value of the first logical storage element; determiningthat the second snapshot metadata does not reference the second currentvalue of the second logical storage element; storing the second value ofthe second logical storage element at a location on the second storagesystem; and modifying the snapshot metadata of the second snapshot toreference the location.